Everyday Science curriculum

Temperature tracks molecular motion: for a gas, it is a measure of the average kinetic energy, or energy of motion, of its molecules. Heat is energy on the move, and on its own it flows from the hotter object to the colder one until both reach the same temperature. How much heat changes hands depends on the temperature gap and on each object's heat capacity, so temperature alone does not tell you how much heat something can give you.

Specific heat is the energy needed to warm a set mass of a substance by one degree. Water's is high: a kilogram of water takes 4,184 joules to warm 1 degree C, while a kilogram of copper takes only 385, so the same heat warms a kilogram of copper nearly eleven times as much. That is why water takes longer to heat up and to cool down than land does, and why it works well as the coolant in a car's radiator.

Conduction is heat flowing between things in contact, always from the warmer to the cooler. Metals conduct heat very well and air conducts it poorly, so a metal surface draws heat out of a warm hand much faster than air at the same temperature. How cold something feels tracks how fast your skin loses heat, the same idea behind the weather service's wind chill, which is based on the rate of heat loss from exposed skin.

Convection moves heat by carrying it along with a moving fluid, such as air or water. Air warmed at a surface forms a warmer, less dense bubble that rises while cooler, denser air moves in to replace it, and in a pot the same effect stirs a boiling liquid. An air fryer, essentially a small countertop convection oven, crisps food by circulating hot air quickly, so overcrowding the basket blocks the airflow and cooks unevenly.

Radiation moves heat as electromagnetic energy that can cross space, which is how the sun's energy reaches Earth. Near a campfire or a fireplace you feel it directly: the side of you facing the fire warms while your other side does not, and the air around you plays no part in that transfer. That sets radiation apart from conduction, which needs objects in contact, and convection, which needs a moving fluid such as air.

Once water is boiling, extra heat does not raise its temperature; it goes into turning liquid into vapor. Energy taken up during a change of state at a constant temperature is called latent heat, and water's is large: vaporizing it takes about five times the energy needed to warm it from freezing to boiling. So a hotter burner cooks boiled food no faster; the water just boils off sooner. A pressure cooker is the exception.

Thermal expansion is the slight growth of a material as it warms, set by its coefficient of thermal expansion: the fraction of its length it gains per degree. Steel grows about 12 millionths of its length per degree C and brass about 24, so a brass part warmed alongside a steel one expands much more and the two may no longer fit together. That is why industrial lengths are defined as a part's size at 20 degrees C.

A microwave oven sends out short radio waves that water, fat, and sugar absorb, setting those molecules moving fast and heating the food. The waves reach only about 1 to 1.5 inches in, so the center of a thick piece cooks by conduction from the outer layers, not from the inside out. Because the oven air stays at room temperature, the surface stays cooler than in a conventional oven heated by hot air, so microwaved food rarely browns or crisps.

A refrigerator cools by moving heat, not by making cold: like any heat pump, it uses electricity to carry heat from a cool space to a warmer one. It pulls heat out of the food compartment and releases it into the room air around it. A heat pump water heater runs the same idea in reverse, moving heat from room air into a tank of water, which is why it tends to cool the room it sits in.

Turning liquid into vapor takes energy to pull apart the bonds between water molecules, so evaporation draws heat from whatever the liquid sits on. An alcohol wipe on your arm feels cold because it evaporates fast and takes heat with it, and sweat cools you the same way. In very humid air sweat evaporates slowly and cools you less, which is also why evaporative coolers, which blow air through a wet pad, work best in dry climates.

Freezer burn shows up as grayish-brown, leathery spots where air has touched the surface of frozen food. The ice there sublimates, meaning it changes directly from solid to vapor without melting first, and the lost water leaves the spot dried out. It harms quality, not safety: the dry parts can be trimmed away, and wrapping that keeps air off the food helps prevent it.

In liquid water, molecules sit in a fairly random arrangement; in ice they line up in a regular lattice that holds them farther apart. So freezing lowers water's density by about 9 percent, and ice floats with roughly a tenth of its volume above the waterline. Most substances are denser as solids. If water were too, ice would sink, and a frozen lake would have no ice on top to skate on.

When food freezes slowly, its water has time to build large, disruptive ice crystals that damage cells and break up emulsions, and the harm shows on thawing: meat drips and loses juiciness, and mayonnaise or cream separates and looks curdled. Freezing fast leaves the molecules no time to form big crystals, which is why food frozen quickly, spread out in a single layer rather than stacked, keeps its quality better.

Small cloud droplets can stay liquid well below 0 degrees C, and a pure droplet may reach about -40 degrees C before freezing; liquid water below freezing is called supercooled. Ice usually starts on a nucleus, such as a dust particle or an existing ice crystal. Where ice crystals and supercooled droplets mix in a cloud, water molecules move from the droplets to the crystals, so the crystals grow while the droplets shrink.

Wind chill is how cold it feels to people and animals outdoors, based on how fast wind and cold draw heat from exposed skin. At 0 degrees F with a 15 mph wind, the wind chill is -19 degrees F. A common misreading is that objects get that cold too: wind only cools a car radiator or water pipe to the actual air temperature sooner, never below it, and frostbite needs the real air temperature to be below freezing.

Frost needs no rain. On a clear night, surfaces lose heat to space, a process called radiational cooling, and light winds let a thin layer of air at the ground grow colder than the air at thermometer height. When that air cools to its frost point, the dew point when it is below freezing, water vapor deposits straight onto plants and cars as ice crystals. That is why frost can appear while the thermometer reads in the mid-30s F.

In a water molecule, both hydrogen atoms sit on one side of the oxygen atom. That leaves the hydrogen side slightly positive and the oxygen side slightly negative, so the positive end of one molecule attracts the negative end of the next. This pull between molecules, called cohesion, makes water sticky and gathers it into drops, and the same charges let water tug salts apart and dissolve them.

Inside water, each molecule is pulled equally in all directions, but molecules at the surface have no water above them, so they cling harder to those beside and below. That surface tension makes the top act like a stretched elastic sheet: it pulls droplets round and can hold up a carefully placed steel paper clip, which sinks once the surface is disturbed. Water striders stay up by spreading their weight on long, water-repelling legs.

Capillary action is water moving through tiny spaces in a porous material, driven by adhesion, water's attraction to other substances, and cohesion, its attraction to itself. Water creeps up a paper towel until gravity balances the pull, and climbs higher in narrow tubes than wide ones. In plants it helps bring water into the roots, but it lifts water only a short way; other forces in the xylem, a plant's water-carrying tubes, do the rest.

Water earns the name universal solvent by dissolving more substances than any other liquid, though not everything. The slightly charged ends of its molecules attract the charged sodium and chloride ions in salt strongly enough to pull them apart and surround each one, which is what dissolving salt means. Oil behaves differently: in water it floats as a separate layer and the two do not mix, as in a bottle of salad dressing.

Water takes far more energy to change temperature than land does, so a nearby ocean or lake warms slowly in summer and cools slowly in winter, and it slows how fast the air nearby changes too. So coastal cities tend to see smaller swings than inland ones. Nebraska's winters are colder and its summers hotter than Oregon's, even though Oregon lies farther north, partly because Oregon has the Pacific next door.

Water is densest not at its freezing point but at about 4 degrees C (39 degrees F). In fall, a lake's cooling surface water grows denser, sinks, and mixes the lake. But water colder than about 4 degrees C is lighter again, so in winter the coldest water stays on top, which is why a lake freezes from the top down. If water were densest at its freezing point, a lake could freeze from the bottom up.

Water hardness is mainly how much calcium and magnesium are dissolved in it, picked up as groundwater moves through soil and rock. When hard water is heated, as in a water heater or coffee maker, calcium carbonate can come out as solid scale that clogs pipes and lowers a heater's efficiency. Soap reacts with the calcium to form soap scum, so you need more soap to get clean. Hardness is a nuisance, not a health concern.

Water's boiling point depends on the air pressure above it. At sea level water boils at 212 degrees F, but each 500 feet of elevation lowers that by just under 1 degree F, so at 7,500 feet it boils at about 198 degrees F. Food boiled or simmered there cooks at that lower temperature and needs more time, and turning up the burner does not help. Raising the pressure reverses this: in a pressure canner, added pressure lets water reach 240 degrees F.

Melting and dissolving can both make a solid seem to vanish, but they are different processes. Melting is a change of state: ice that gains heat becomes liquid water, as an ice cube does in a cold drink. Dissolving needs a second substance: when salt goes into water, water molecules pull its sodium and chloride ions apart and surround them, giving an even mixture, a solution, that still contains the salt.

Pure water freezes at 32 degrees F, but dissolving salt in it lowers that freezing point, and more salt lowers it further. Road salt dissolves into water as charged particles called ions, so ice can melt into salty water without getting any warmer. There is a floor: a sodium chloride solution freezes no lower than about -6 degrees F. Applied before ice bonds to pavement, salt stops that bond from forming, so the road stays wet or slushy.

Cream will not freeze in plain ice water, because milk's freezing point is below 32 degrees F, the temperature of plain melting ice. Adding salt lowers the freezing point of the ice's meltwater, so the ice keeps melting below 32 degrees F. Melting absorbs heat, which the cold, salty bath pulls from the cream until it freezes; more salt freezes it faster. Nearly anything that dissolves in water lowers its freezing point.

Warm water holds less dissolved gas, such as oxygen or carbon dioxide, than cold water. A cold soda keeps more carbon dioxide in solution than a warm one, so it tastes fizzier, and warm streams may carry too little oxygen for some fish and other aquatic life. Solids vary more: hotter water can dissolve more minerals from the rock it touches, yet heating hard water makes calcium carbonate come out as scale.

Sodium acetate, a salt used in heating pads, dissolves fully in hot water and stays dissolved as the solution cools, leaving it supersaturated: holding more dissolved salt than a cool solution normally would. It stays liquid until something, such as strong shaking or a jab from a sharp tip, starts the first crystals. Then the clear liquid turns into an opaque, milky solid and gives off heat.

Osmosis is water crossing a membrane that lets water through but holds back dissolved salt, moving from the less salty side toward the saltier side. Salt on meat or fish draws water out of the tissue this way. In dry brining, poultry rubbed with salt and spices rests in the refrigerator for up to two days; the moisture drawn out forms a flavorful brine that the meat then reabsorbs, seasoning it without added water.

Oil and water do not mix: left alone, oil floats in its own layer, as in a bottle of salad dressing. An emulsifier is an ingredient that lets the two blend smoothly and keeps them from separating, and food labels list examples such as egg yolks, soy lecithin, and mono- and diglycerides. Emulsions can still break: in slowly frozen food, large ice crystals break emulsions such as mayonnaise or cream on thawing, leaving them curdled.

Atmospheric pressure is set by the mass of the column of air above a spot, pulled down by gravity. At sea level that weight presses about 14.7 pounds on each square inch of you, roughly a ton on each square foot. You do not feel crushed because the fluids and air inside your body push outward with the same pressure. Changes are what you notice: dive a few feet underwater and you feel extra pressure on your eardrums.

Air pressure drops with height because less air sits above you; half of the atmosphere's molecules lie below about 18,000 feet. At sea level air presses 14.7 pounds per square inch, but about 12.3 at 5,000 feet. On the way up, air in your middle ear can end up at higher pressure than the air outside, pushing the eardrum out until it evens out with a pop. Oxygen stays the same share of the air, but each breath holds fewer oxygen molecules.

Boyle's law says that for a trapped gas held at a constant temperature, pressure times volume stays about the same. Squeeze the gas into three quarters of its space and its pressure rises by about a third; let it expand and the pressure falls. Breathing runs on a pressure difference of this kind: chest muscles tighten to make a slight vacuum around the lungs, so outside air flows in, and when they relax the lungs deflate like an open balloon.

Air pressure in a closed container comes from molecules striking its walls. Warm the air and the molecules move faster and hit harder, so pressure rises; cool it and pressure falls. A tire follows the same rule, which is why the recommended pressure is a cold figure, meaning the car has not been driven for at least three hours. A reading from a warm tire includes extra pressure from the heat, so it is not a fair comparison.

Relative humidity is the moisture in the air as a percentage of the amount present if the air were saturated. Because that saturated amount depends on temperature, the same moisture gives a lower reading in warmer air: with a dew point of 45 degrees F, humidity is 100 percent when the air is 45 degrees F but only 34 percent at 75 degrees F. The dew point, the temperature air must be cooled to for saturation, says more about actual moisture.

Sucking on a straw removes air from inside it, lowering the pressure there. The atmosphere still presses on the open surface of your drink, about 14.7 pounds per square inch at sea level, and that outside push forces the liquid up the tube. A mercury barometer shows the same thing: air pressing on a bowl of mercury drives it about 30 inches up a tube with a vacuum at the top. The push comes from outside.

A suction pump does not pull water up; it removes air from its pipe and lets atmospheric pressure push water in. At sea level that pressure, about 14.7 pounds per square inch, can hold up a water column at most 33.9 feet tall, and only with a perfect vacuum. Real pumps never reach one, so a good fire pump lifts water about 22 feet. In 1643, trying to explain this limit, Evangelista Torricelli made a barometer using mercury.

Carbonated drinks are made by dissolving carbon dioxide into them under pressure, and they stay fizzy only while kept under pressure. Opening the bottle releases that pressure, so the dissolved gas comes out of solution as bubbles and sometimes foams over. A cold soda also keeps more carbon dioxide dissolved than a warm one. Many volcanic eruptions work in a similar way, as gases dissolved in rising magma come out when the pressure drops.

pH tells you how acidic or basic a water solution is, on a scale from 0 to 14 with 7 as neutral: more free hydrogen ions make it acidic, more hydroxide ions make it basic. The scale is logarithmic, so each whole step is a tenfold change. pH 5 is ten times as acidic as pH 6, and three steps apart is a thousandfold gap. Even ordinary rain, near pH 5.6, is slightly acidic.

Baking soda is sodium bicarbonate, an alkaline compound. When it meets an acid, the two react and cancel each other out, a change called neutralization, and the reaction releases carbon dioxide: the bubbles you see when it hits vinegar. Bicarbonate does the same job in nature, where it helps a lake take in acid rain or an acid spill without a sharp drop in pH.

A soap molecule has a polar head that dissolves in water and a nonpolar tail that mixes with oil. In water, soap gathers into tiny clusters called micelles, heads out and tails in, and their oily cores hold greasy dirt. Lathering and scrubbing add friction that loosens soil and microbes, and rinsing washes them off. Much of the benefit is removal, though soap can also break apart viruses that have an oily outer envelope.

Surface tension comes from water molecules attracting one another at the surface. A surfactant, short for surface-active agent, lowers it; every soap is a surfactant, though not every surfactant is a soap. A liquid wets a surface, spreading into a thin film, when its surface tension is low and the surface attracts it. So soapy water can spread where plain water would stay as a rounded drop.

Cleaning with soap or detergent, water, and scrubbing removes most dirt and germs from a surface. Disinfecting kills most germs, and household bleach, a solution of sodium hypochlorite, is a common disinfectant: the compound acts as an oxidizing agent and is also used to bleach. The jobs come in order, because dirt left behind makes it harder for a disinfectant to reach and kill germs.

Household bleach is a solution of sodium hypochlorite, and it reacts with some other cleaning chemicals. Combined with ammonia, it gives off chloramines, gases that are very hazardous to breathe. Combined with acids, such as some toilet cleaners, it releases chlorine gas, which reacts with the moist lining of the airways and irritates them. Chlorine is heavier than air, so it is most dangerous in enclosed or low-lying spaces.

Hard water is water carrying a lot of dissolved calcium and magnesium, picked up as it moves through soil and rock. Soap reacts with that calcium to form soap scum, the residue that can leave your hands feeling filmy after washing. In hard water you also need more soap or detergent to clean hands, hair, or laundry, and it leaves spots and film on dishes and glasses.

Many detergents carry enzymes, each of which breaks a specific kind of chemical bond. Proteases cut the bonds that hold proteins together, so they lift egg, blood, and grass stains. Amylases split starch from foods like rice and potatoes, and lipases break down fats such as butter and skin oils. Because enzymes do this work, these detergents can clean at lower water temperatures, and cold-active enzymes are drawing interest for cold washes.

Fire is a very fast chemical reaction between oxygen and something that burns, giving off heat, light, and smoke. Take away any one of the three ingredients and it goes out. Water cools away the heat; dirt, or rolling on the ground, smothers it by cutting off oxygen; and firefighters dig a fireline to clear fuel from a fire's path. Some extinguishers also work on a fourth element, the chemical reaction itself.

Around a candle, warm, less dense air rises and cooler air flows in from below, stretching the flame upward into its familiar shape. That flow also sweeps soot to the flame's tip, which makes the flame yellow. In microgravity, where warm air does not rise, a candle flame forms a sphere and burns blue with no soot. On the space station, a sooty yellow flame on a solid fuel grew bluer and shrank as the air pressure was lowered.

Rusting is a reaction in which iron atoms lose electrons, a step called oxidation, while dissolved oxygen takes those electrons up. In perfectly dry air it barely happens; corrosion runs whenever a thin layer of water forms on the metal and acts as an electrolyte, a liquid that carries current. Dissolved salt makes water conduct far better, and chloride from sea spray is one of the commonest corrosive agents in air.

Join zinc to a less active metal in a liquid that conducts and the zinc becomes the anode: its corrosion speeds up while the other metal's slows or even stops. This is called cathodic protection. Boats can carry zinc anodes welded to the hull for this reason, and zinc-rich primers on steel bridges protect any bare steel exposed at defects. The farther apart two metals sit in activity, the stronger the effect.

A battery has two terminals, the cathode and the anode, kept apart by an electrolyte, a material that ions (atoms or molecules carrying an electric charge) can move through. When the battery runs a device, electrons travel through the outside circuit while ions cross the electrolyte to balance their charge. A battery does not hold electricity as such: charging stores the energy as chemical potential, and discharging turns it back into current.

At low temperature the chemical reactions in a lithium-ion cell run less efficiently, which lowers its output voltage and the capacity it can deliver. The electrolyte also thickens, so lithium ions move sluggishly between the electrodes. Many electric cars carry battery heaters to offset part of this loss. In DOE vehicle tests, though, the bigger winter drain on range was heating the cabin, with battery chemistry second.

In an intact apple, the browning enzyme, polyphenol oxidase, is kept apart from the phenolic compounds it acts on by cell compartments. Cutting or bruising breaks that separation, and with oxygen present the enzyme turns phenols into quinones, which link up into brown pigments called melanins. Acids slow this by lowering the pH, which deactivates the enzyme; vitamin C, or ascorbic acid, helps too, though often only for a while.

Class A fires burn ordinary solids such as wood, paper, and cloth; Class B, flammable liquids and grease; Class C, live electrical equipment; Class D, combustible metals; Class K, cooking oils and fats. Water cools fuel and suits Class A, but on burning liquid it can spread the fire, on wiring it risks electrocution, and on a deep fryer it can trigger an explosive reaction. Carbon dioxide displaces oxygen; dry chemical coats the fuel.

The Maillard reaction is browning between reducing sugars such as glucose and the amino acids of proteins, with no enzyme involved. It is named for Louis Camille Maillard, who in 1912 saw a heated solution of a sugar and an amino acid turn brown. Its products help give bread, coffee, and chocolate their color and flavor. Heat speeds it up sharply, but it is not limited to high temperatures: it creeps along slowly even in frozen meat.

Heat sugar on its own and the white crystals turn into brown caramel with a slightly bitter, burnt-sugar flavor. That is caramelization: the sugar molecules themselves rearrange and break down, with no protein needed. The Maillard reaction instead joins sugars with amino acids. Neither uses an enzyme, and both help brown a bread crust, where they compete for the same sugars.

At standard air pressure, liquid water turns to vapor above 100 degrees C, so food sitting in it stays at or below that temperature, and its surface stays wet. The Maillard reaction speeds up as temperature rises, and too much water dilutes the sugars and amino acids it needs. A baking bread crust, by contrast, dries out and browns at around 160 degrees C. Boiled food is not immune to browning, only very slow at it.

Baked goods rise on gas. Yeast makes it biologically, breaking down carbohydrates into carbon dioxide and ethanol, which takes time. Baking soda, sodium bicarbonate, releases carbon dioxide quickly when it meets an acid, such as the acid in yogurt whey; used alone it can leave an unpleasant taste. Baking powder combines the bicarbonate with acidic salts plus starch, which absorbs moisture so the two do not react too early.

Denaturing a protein unfolds it from its natural shape. Heated egg proteins unfold and bond to one another in a permanent three-dimensional network, so a cooked egg white cannot turn back into a liquid. Beating egg whites unfolds proteins too: they line up where air meets water and form a thin film around each bubble, which holds the foam. Whites foam best at room temperature; too much heat can denature the proteins and collapse the foam.

In gelatinization, heat and water break down the ordered regions inside starch granules, so the granules soak up water and swell, and the mixture grows thicker. During storage, cooked starch slowly rearranges into more ordered structures, a process called retrogradation, which contributes to bread going stale. The crust stales differently: it turns leathery, which is thought to come from moisture moving from the crumb into the crust.

Reactions such as browning run faster as temperature rises, and bacteria multiply faster warm than chilled: at room temperature, the bacteria that cause foodborne illness can double every 20 minutes. Keeping a refrigerator at 40 degrees F or below slows that growth, and slow chemical changes still continue in cold storage. The bacteria that make you sick differ from those that cause spoilage, so food can look and smell fine yet still be unsafe.

An onion makes its tear-inducing chemical only when its tissue is damaged. Cutting lets the enzyme alliinase break down a sulfur-containing compound into a sulfenic acid, and a second enzyme, lachrymatory factor synthase, turns that into propanethial S-oxide, the volatile that brings tears. It is thought to defend the plant against herbivores and microbes. In one study, onions bred from irradiated seed caused no tears; they had little alliinase.

Light travels as a wave, yet it also comes in discrete packets of energy called photons, which have no mass. Which side you see depends on the detector. A spectrometer that spreads light into a spectrum by diffraction is reading the wave side; a digital camera sensor, where single photons free electrons to record the picture, is reading the particle side.

Human eyes typically detect wavelengths from about 380 nanometers, seen as violet, to about 700 nanometers, seen as red; a nanometer is a billionth of a meter. Cone cells in the eye are tuned to this narrow band. The same spectrum runs on to waves smaller than an atom and to radio waves longer than Earth is wide, and you can see none of them.

A surface looks colored because its material absorbs some wavelengths and returns the others. Leaves look green because chlorophyll takes in red and blue light and reflects green. Much of this happens just under the surface: light enters a rose petal, reflects around inside, and loses its green before it comes back out red. A surface that absorbs almost everything, like black pavement, looks dark and gets hot.

Mixed lights add their wavelengths: red, green, and blue light can be combined into most visible colors, and blue and yellow lights together look white. Pigments work the other way, because each one absorbs part of the spectrum. Blue paint swallows the reds and yellows, yellow paint swallows the blues and violets, and green, the one band both let through, is all that comes back. Paint mixing and light mixing are different processes.

Nitrogen and oxygen molecules are much smaller than light waves, and particles that small scatter short wavelengths far more strongly than long ones, a process called Rayleigh scattering. Blue light scatters about four times as strongly as red, so it is redirected all over the sky and reaches you from every direction. Violet scatters even more, but your eyes are more sensitive to blue, so blue is what you see.

Near sunrise and sunset, sunlight takes a far longer path through the atmosphere, and along the way so much violet and blue is scattered out of the beam that the light reaching you is reddened. The red of sunset is the leftover of the same scattering that makes the midday sky blue. A common belief is that dust and smog brighten sunsets, but haze in the lower air usually mutes them; clean air is the common ingredient of vivid ones.

Air molecules are far smaller than light waves and favor blue. Cloud droplets, about 0.02 millimeter across, are dozens of times wider than visible light waves, and scattering by drops that size, called Mie scattering, treats all colors about the same, so the light a cloud sends out stays a mix that looks white. Light reflects again and again inside a cloud, and as a cloud thickens, less sunlight gets through and it turns gray.

Fluorescence is light a material gives off while it is being excited, at a longer wavelength than the light that excites it. Often ultraviolet you cannot see goes in and visible light comes out. A glow that continues after the exciting light stops is called phosphorescence instead. Laundry brighteners rely on fluorescence: they absorb ultraviolet and emit visible light, so treated fabric looks whiter and glows under a black light.

Measured from a line perpendicular to the surface, a reflected ray leaves at the same angle at which it arrived, and both rays lie in one plane with that line. A very smooth surface such as a mirror reflects almost all the light that strikes it. The reflected rays spread out as if they came from a point behind the glass, so that is where you see the image: the same distance back from the mirror as the object stands in front.

A ray that meets a flat mirror head-on bounces straight back along its own line, and each image sits as far behind the glass as its object is in front. Apply that to yourself: the image of your raised left hand is directly across the glass from that hand, and your nose, being nearer the glass than the back of your head, has its image nearer too, so your reflection faces you. What the mirror reverses is the direction toward and away from it.

Light moves more slowly in water than in air, and when it crosses the surface at a slant that change in speed bends its path; a ray meeting the surface straight on passes through unbent. Your eye places objects along the bent rays traced straight back, so the underwater part of a straw or an oar seems shifted and looks broken at the waterline. The same bending makes a lake bottom look shallower and a fish look nearer than it really is.

From water or glass into air, light bends away from the perpendicular, and past a certain slant, called the critical or limiting angle, it cannot get out: the whole beam reflects back inside. This only happens when light heads into a less refracting medium. The limit is about 48.5 degrees for water and shrinks as a material bends light more strongly, so diamond traps light easily and, with its strong spreading of colors, gets its brilliance.

A convex lens, one that bulges outward, makes parallel rays converge to a point; a concave lens, one that curves inward, spreads them apart. Focal length is the distance from the lens to the image of a far-off object. A camera lens casts a real, upside-down image. A magnifying glass is a short-focus lens held slightly closer to an object than its focal length, giving an enlarged, upright image. In your eye, the cornea and lens do the focusing.

In nearsightedness the eye focuses light in front of the retina, the light-sensing layer at the back of the eye, so distant things blur; in farsightedness it focuses behind the retina, so close things blur. Causes include an eyeball that is too long or too short and a cornea of the wrong shape. A glasses lens bends light so it focuses on the retina, a concave lens for nearsightedness. Glasses do not make your eyes weaker.

Polarization describes how the electric field of a light wave is lined up. Much of the light reflected from a smooth surface such as glass or water is polarized, vibrating mostly parallel to that surface, and at one particular angle, which is larger for materials that bend light more strongly, all of it is. Polarized sunglasses work like a picket fence that lets one alignment through and stops the other, absorbing the polarized glare.

Each raindrop bends sunlight on the way in, reflects it off its curved back, and bends it again on the way out. Much of that light bunches along a path at about 42 degrees, and because each color bends a slightly different amount the colors separate, red on the outside of the bow and blue on the inside. You see one only with the sun behind you and rain ahead, and none at all when the sun is more than 42 degrees high.

Not all the light leaves a raindrop after one internal reflection; part of it reflects a second time and exits at a different angle, about 50 degrees instead of the primary bow's 42. That light forms a secondary rainbow, and the extra reflection reverses its colors, so its order runs opposite to the main bow's. Light can reflect more than twice inside a drop, but such extra bows are almost never seen under ordinary conditions.

A halo is a ring of light around the sun or moon that forms when its light refracts through ice crystals in a thin veil of cirrus cloud; it usually looks white but can show color. Cirrostratus, a thin, veil-like sheet of cloud, nearly always produces one. Sun dogs, also called mock suns or parhelia, are colored spots of light about 22 degrees to the left or right of the sun, red on the side nearest the sun and blue farther out.

When the sun bakes a stretch of sand, the air touching it becomes lighter and bends light less than the cooler air above, so the air grows denser with height. Light from a distant object heading down at a very shallow angle is bent back upward as it passes through this layer, so it reaches your eye from below and you see an image much like a reflection in a pool. There is no water there; such images tantalized thirsty French soldiers in Egypt.

Photos taken with the same zoom show the rising moon is as wide as the moon high in the sky. If anything, the air squashes the low moon a little vertically, and it is about 1.5 percent farther away. Hold your outstretched index finger beside the low moon: its fingernail and the moon look about the same size. Why the brain enlarges it is still unsettled: depth cues from the landscape are one idea, but astronauts in orbit see the illusion too.

Twilight is the time before sunrise and after sunset when the upper atmosphere, still lit by the sun, scatters sunlight down to the ground. It is divided by the sun's depth below the horizon: civil twilight ends at 6 degrees, after which outdoor tasks normally need lamps; nautical twilight ends at 12, while sailors can still see the horizon; and astronomical twilight ends at 18, when scattered sunlight becomes fainter than starlight.

Turbulent air holds small pockets that differ in density and temperature, and as they drift across your line of sight they bend a star's light this way and that, so its brightness and position seem to jump. Stars near the horizon twinkle more because their light crosses more air. Planets can look like points too, but they are wider than this blurring, so their light passes through many pockets at once and the effects average out.

Light, electromagnetic waves, and electromagnetic radiation name one thing, and the familiar bands are slices of a single spectrum. Radio waves and microwaves, for example, differ only in wavelength, the distance between crests. Shorter waves carry more energy per photon, rising from radio through visible light and ultraviolet to X-rays and gamma rays. Radio is usually labeled by frequency, visible light by wavelength, and X-rays by energy.

An oven's microwaves bounce off its metal walls, pass through glass, paper, ceramic, and plastic, and are absorbed by food, where they make water molecules vibrate and produce heat; watery foods such as fresh vegetables cook quickly. A microwave-safe dish is not heated by the waves, but it can get hot from the food in it. Thick foods are not cooked from the inside out; heat reaches the middle by conduction from the hot outer layers.

Microwaves reflecting off the metal walls overlap into a standing wave, a pattern that stays in place, with spots of strong and weak heating. Food left still heats unevenly, with hot spots and cold spots, and its shape adds to it: the corners of a rectangular portion tend to run hot. A turntable carries the food through the pattern, and turning the dish by hand and stirring partway through help even out the heat, though cold spots can remain.

Infrared lies just past the red end of the visible spectrum; your eyes cannot see it, but you can feel some of it as heat. Very hot objects such as a fire also glow visibly, while cooler ones such as people give off only infrared. Thermal infrared cameras sense that emission, so they can show a warm animal in the dark. Many surfaces also reflect infrared, especially near-infrared, so not every infrared image is a heat picture.

Ultraviolet has shorter wavelengths than visible light, and more energy per photon. Sunlight's ultraviolet is divided into three bands, UV-A, UV-B, and UV-C. UV-C is the most harmful, but the atmosphere absorbs almost all of it, and ozone absorbs about 95 percent of UV-B, the band that causes sunburn and raises the risk of DNA and other cell damage. Too much ultraviolet can also harm the eyes, age the skin early, and cause skin cancer.

Glass filters ultraviolet unevenly. Plain glass blocks almost all UV-B but only about a fifth of UV-A. Laminated glass, made of glass layers bonded around a plastic sheet and used in windshields, blocks nearly all UV-A too. In a 2025 test of 34 recent cars, windshields blocked about 99 percent of UV-A and driver side windows about 89 percent. So a window cuts the band that causes sunburn, but some UV-A still gets through.

A brick wall stops visible light, yet radio signals still reach receivers inside buildings, only weaker. Across many measured buildings the loss was usually 10 to 30 decibels and sometimes 80 or more. Frequency matters too: in one 60-story office tower a 750 MHz signal lost about 12.8 decibels less than a 2.4 GHz signal, and walls weaken 60 GHz signals so much that such networks are expected to stay within a single room.

Ionizing radiation, such as X-rays, gamma rays, and the alpha and beta particles of radioactive decay, can strip electrons from atoms, which can alter molecules and damage cells. Along the light spectrum the dividing line falls within the ultraviolet, and the ultraviolet that reaches the ground is non-ionizing. Visible, microwave, and radio photons carry too little energy each. Microwaves can heat tissue but do not make food radioactive.

Sound is a mechanical wave: a vibration that travels through a medium such as air or water as molecules bump into their neighbors, leaving squeezed and stretched regions in the air as it passes. The energy moves on while the molecules stay roughly in place. With no molecules to pass it along, sound cannot cross the vacuum of space. Light is different: its linked electric and magnetic fields need no medium at all.

You see lightning almost at once, but its thunder moves at the speed of sound, roughly 1,100 feet per second, which takes about 5 seconds per mile, or roughly 3 seconds per kilometer. Count the seconds from flash to thunder and divide by 5 for miles: 15 seconds means about 3 miles. Thunder carries only about 10 miles, so if you can hear it at all, the storm is within striking distance.

Pitch tracks frequency, the number of vibrations per second, and a young, healthy ear hears roughly 20 to 20,000 of them. Loudness depends mainly on amplitude, how wide each vibration swings, though frequency also affects how loud a sound seems. A struck tuning fork shows that the two are separate: as it fades, its prongs swing less and the sound grows quieter, but its pitch stays the same.

The decibel scale is logarithmic, so you cannot add decibel readings like ordinary numbers. Doubling the sound power raises the level by only about 3 decibels, which means two equally loud machines together read about 3 decibels more than one, not twice as many. Turned around, every 3-decibel step doubles the energy reaching your ears, and long or repeated exposure at or above 85 A-weighted decibels can cause hearing loss.

Resonance is the buildup of vibration when pushes arrive at an object's own natural rate. Two tuning forks with the same note show it: sound one, and the other starts to hum on its own, because each faint push through the air lands in step and the motion adds up. Stick a small coin on one fork and the match is lost, so the response stops. A water carrier whose steps match the sloshing can build waves until the water spills.

When a sound source moves toward you, its waves are compressed, so they arrive at a higher frequency and the pitch sounds higher. As it moves away, the waves are stretched and the frequency drops. That is why a siren or a train whistle seems to slide lower just as the vehicle passes you. Doppler weather radar relies on a similar shift in its returning radio pulses to tell whether raindrops are moving toward it or away.

Sound hitting a wall is partly reflected, partly transmitted, and partly absorbed. A hard, smooth wall sends back most of it, so in a bare room sound bounces many times before dying away; this slow fading, called reverberation, blurs speech. Porous materials such as carpets and curtains absorb sound and shorten it. An echo is a distinct repeat from a distant surface, heard separately when it lags by at least about a fifteenth of a second.

While you talk, you hear your voice through the air and also through vibrations carried by bone, called bone conduction, which reach only you. A recording keeps just the airborne part. Researchers name that missing sound as a main reason recordings seem odd; bone conduction is thought to favor low frequencies. In one study people told their own voice from others' better when it was played through bone, though unfamiliarity may also matter.

Active noise-canceling headphones use a microphone to pick up the sound around you, then play anti-noise: a matching signal of the same size but opposite phase, so it pushes when the noise pulls. Where the two overlap they cancel, which is called destructive interference. Current systems handle low-frequency noise, below about 1,000 hertz, well and higher pitches less well, possibly because those are harder to estimate accurately at the ear.

Voltage, also called potential difference, is the push that drives electrons through a conductor. Current is the movement of those electrons, and resistance, measured in ohms, is the opposition to it. Ohm's law ties the three together: voltage equals current times resistance. So at a fixed voltage, a lower resistance lets more current flow, and a higher one lets less. For example, 12 volts across 6 ohms drives 2 amps.

A watt measures power, the rate of use at a given moment, and a kilowatt is 1,000 watts. Energy used over a stretch of time is counted in watt-hours: one watt-hour is one watt used for one hour. Utilities generally measure what customers use in kilowatt-hours. A 40-watt bulb left on for five hours uses 200 watt-hours, or 0.2 kilowatt-hours. Watts tell you how fast; kilowatt-hours tell you how much.

In a series circuit, current has only one path, so the same current flows through every part, and one break, such as an open switch or a burned-out part, stops current everywhere. In a parallel circuit, each component is connected across the same voltage source on its own branch, and every branch sees the same voltage. Opening one branch stops only that branch, while current keeps flowing through the others.

Too much current overheats wiring, so circuits carry devices that open automatically when current runs too high. A fuse holds a metal link sized so that normal current leaves it intact, but an overload or short circuit melts it, and the fuse must then be replaced. A breaker trips instead: in lower-current breakers, a bimetal strip bends from the heat of excess current and releases the contacts, and the breaker can be reset.

Electricity flows in closed circuits, and you get a shock when your body becomes part of one, for example by touching a live wire while also touching the ground. Grounding means deliberately giving current a low-resistance path to earth. If a fault energizes a tool's metal case, the equipment ground carries that current away and can trip the circuit protection. It sharply lowers the risk of shock, but it does not guarantee safety.

Current needs a complete path, and a body gets shocked when it links two wires of a circuit, or a live wire and the ground. A bird perched on a single line and touching nothing else completes no such path, so it is not shocked. Birds are electrocuted when, perching where clearance is too small, they touch two wires carrying different phases at once, or a live wire and a ground wire together.

Direct current runs steadily in one direction, as in a battery or fuel cell, and early on its voltage could not easily be raised or lowered. Alternating current reverses direction many times every second, and a transformer can change its voltage with relative ease, which made long-distance transmission cheaper. The grid still runs mainly on AC, but computers, LEDs, and electric vehicles run on DC.

Standby power is what a device draws in its lowest-power mode, typically when it looks switched off or is not doing its main job. TVs, computers, and kitchen appliances that stay plugged in are common examples. By one national laboratory estimate, a typical American home has about forty products drawing power constantly, together almost 10 percent of residential electricity use. Unplugging the device stops the draw.

Every magnet has a north pole and a south pole. Push two norths or two souths together and they shove each other away, while a north and a south snap together. The field can be pictured as lines of force that leave the magnet at its north-seeking pole and come back in at its south-seeking pole. The bundle of field lines leaving the north pole is called magnetic flux.

Each spinning electron acts as a tiny magnet, but in most materials the spins point every which way and cancel. In ferromagnetic metals such as iron, steel, nickel, and cobalt, electrons in small regions called domains spin the same way, and the domains line up easily, so a magnet pulls them strongly. Aluminum concentrates a field only slightly more than empty space does, and copper, gold, and silver less, so they are only weakly magnetic.

The Danish scientist Oersted found that a wire carrying current is surrounded by a magnetic field. Wind the wire into a coil and the field lines crowd together inside it, all pointing the same way, so the coil acts like a magnet. Its strength grows with the current and with the number of turns, and an iron core slipped inside concentrates the field further. A magnet made this way, by electricity, is called an electromagnet.

A wire feels a force when two things are true at once: it carries current, and it sits in a magnetic field. The push is at right angles to the field. In a motor the wire is bent into a loop on a rotor, so its two sides are pushed in opposite directions and the rotor turns. A stronger field or more current gives more torque, or turning force, and reversing the current reverses the direction of that torque.

Michael Faraday showed in 1831 that a magnet moved inside a coil of wire induces a current in it. That takes a magnetic field, a conductor, and relative motion between them, which includes a field growing or collapsing past the wire; a still loop in a steady field produces nothing. A stronger field, or faster cutting, raises the voltage. Generators like this make nearly all US electricity; solar panels are an exception.

When one coil's magnetic field expands and collapses, its field lines cut the windings of a nearby coil and induce a voltage there, an effect called mutual induction. In a transformer, the ratio of the two coils' voltages equals the ratio of their turns, so extra turns on the output side step the voltage up. A transformer cannot pass steady direct current. Substations raise voltage for long-distance lines and lower it for homes.

A gas or ordinary electric burner heats a pan from outside, by flame or a hot element. An induction cooktop instead uses an electromagnetic field to set up electric currents in the cookware, so the heat is made inside the pan, and it stops as soon as the pan is lifted off. The glass top warms only as much as any surface does under a hot pan. Flat-bottomed pots and pans that a magnet sticks to will work.

Earth's core generates most of the magnetic field measured at the surface, though it is far too hot to be a permanent magnet. A compass needle lines up with the horizontal part of that field where you stand, not with a single point such as a magnetic pole. At most places this differs from true north by an angle called declination, which varies with place and time, so a topographic map's printed value holds for the year it was made.

Modulation means putting sound or another signal onto a transmitted radio wave, which has two defining traits: its amplitude, or strength, and its frequency. In amplitude modulation (AM), the sound controls the wave's strength while its frequency stays fixed. In frequency modulation (FM), the sound shifts the frequency while the strength stays constant. A radio tunes to one frequency and turns what it receives back into sound through the speaker.

Static from electrical equipment and the atmosphere is mostly changes in amplitude, a radio wave's strength. AM carries its sound in that very strength, so noise mixes into the program. FM carries sound in frequency changes over a wide channel, which gives it a built-in edge against noise, and an FM receiver tends to lock onto the strongest station. FM stations, on a newer, higher band, did not match the long reach of high-powered AM stations.

Many home routers broadcast two Wi-Fi signals. The 2.4 GHz band generally covers a wider area but carries data less quickly, and it can be crowded, since many household devices and most routers use it. The 5 GHz band is faster and may be less congested by neighboring networks, but its signal does not reach as far. One practical split is to keep the 5 GHz network for the uses that matter most, like work or school.

Each GPS satellite carries an atomic clock and broadcasts the time its signal was sent. Your receiver compares that with the arrival time; since the signal travels about 186,000 miles per second, the delay gives the distance. Distances to three satellites fix a position only if the receiver also had an atomic clock, so a fourth measurement cancels the receiver's clock error. With four, it computes latitude, longitude, altitude, and time.

Every color on a TV or computer screen is a blend of red, green, and blue light in varying amounts, often shortened to RGB. Each pixel can show many levels of each of the three, and those combinations let it show subtle differences in shade. Old picture-tube televisions mixed the same three primary colors at varying intensities, driven by electron guns at the back of the tube.

A screen looks steady when it updates fast enough, because perception blends light arriving in quick succession: an image and its negative alternated rapidly look gray. The needed rate, the critical flicker fusion rate, is usually reported at 50 to 90 Hz (cycles per second). But in a ten-person study of a display swapping a sharp-edged image with its negative, viewers saw flicker artifacts at 500 Hz and up, likely during quick eye movements.

A capacitive touchscreen senses the small change a finger makes in the capacitance at the screen's electrodes, so even a light touch registers. It responds only to things that can form that capacitance, such as fingers or a stylus with a conductive tip, and water drops can be mistaken for touches. Resistive screens instead need a press that pushes two layers into contact, but work with anything.

Inertia is a body's resistance to any change in its motion: something moving keeps going straight at the same speed unless a net force acts on it. When a car stops suddenly, an unbelted occupant keeps going and can strike the interior or other passengers or be thrown out, which is almost always deadly. A belt grips the pelvis, collarbone, and rib cage early in the crash, tying you to the slowing car.

Newton's second law says force equals mass times acceleration: the acceleration an object gets grows with the force on it and shrinks as its mass grows. Push an empty cart and a fully loaded one equally hard, and the loaded cart speeds up less. Slowing down is a change in velocity too, so stopping the loaded cart in the same time takes a bigger force, and the same force takes longer to stop it.

Newton's third law says forces come in pairs: when one body pushes on another, the second pushes back on the first, equally hard and in the opposite direction. A pencil presses down on a desk, and the desk presses up on the pencil. Balance is judged by adding the forces on one object, and each member of a pair acts on a different object. A resting book balances because two forces act on the book: its weight and the table's push.

Momentum is mass times velocity, and the average force on an object equals its change in momentum divided by the time the change takes. Stopping from a given speed removes the same momentum however it happens, so a longer stop needs less average force. A seat belt load limiter caps the belt's pull: once the force on the chest reaches a set level, the belt spools out in a controlled way, holding a steady force as it absorbs energy.

Even at steady speed, an object going around a curve is accelerating, because its direction keeps changing, and acceleration needs a net force. That inward force is called centripetal force; without it, the object moves in a straight line. The outward pull you feel in a car on a curve, called centrifugal force, is only apparent: cut the string on a whirled model plane and it flies off along a tangent, not straight outward.

Static friction holds two surfaces still until a push exceeds its maximum; once sliding starts, friction drops to a lower kinetic, or sliding, value, because the surfaces' tiny bumps interlock less while moving. A wheel that locks and skids loses the greater grip of static friction and is left with weaker kinetic friction. Anti-lock brakes sense a wheel locking and briefly ease the brake so it can roll and regain that grip.

A falling object feels two forces: its weight pulling down and air drag pushing back. Drag grows with the square of speed, so as the object speeds up, drag climbs until it equals the weight. The net force is then zero, the acceleration stops, and the object falls at a constant speed called terminal velocity. Shape and size matter: a large flat plate falls more slowly than a small ball of the same weight.

An object's weight is its mass times g, the acceleration due to gravity, and Newton's second law says acceleration is force divided by mass. Divide the weight by the mass and the mass cancels, leaving g, about 9.8 meters per second squared at Earth's surface. So with no air, a beach ball and an airliner fall together. In air, drag separates them, and the tale of Galileo dropping cannonballs from the Leaning Tower is probably only a legend.

The turning effect of a force, called torque or moment, equals the force times its perpendicular distance from the pivot. A wrench works this way: the push on the end of the handle, times the handle's length, is what turns the nut, so a longer handle needs less push for the same torque. A lever balances when force times distance is equal on both sides, so a force twice as large balances at half the distance.

Torque measures how strongly a force turns something around a pivot: the force times the distance from the pivot, measured at right angles to the force's direction. Push on a door's edge straight toward the hinge and that distance is zero, so the torque is zero and the door does not move, however hard you push. The same push made at right angles to the door, far from the hinge, has a long lever arm and a large torque.

Work is force times the distance moved along the force, and a machine cannot create energy, so cutting the force means pushing through a longer distance. To raise a load by a ramp's height, you push it along the ramp's whole length. In a block and tackle the load hangs from several rope sections, each carrying your pull, so six sections ideally let a 50-pound pull hold 300 pounds, but you must haul six feet of rope to lift it one foot.

A gear train can trade speed for force. When gears slow the rotation from input to output, the output turns with more torque, or turning force. Motorized valve actuators rely on this, using a gear train to cut a fast motor's speed and raise the torque at the valve stem. Large hand-operated valves get the same benefit from geared handwheels, which give mechanical advantage at the cost of speed, so turning them takes longer.

Water cannot transmit much sideways force, so on a wet road a tire can steer and brake only through the parts of its footprint still touching the pavement. Tread grooves are escape channels that let trapped water out from under the tire, raising the depth of water needed before the tire hydroplanes, riding on water instead of road. That benefit fades as tread wears and is gone at about 1/16 inch, the point where tires should be replaced.

Pressure is the force acting on each unit of area, such as pounds per square inch (psi). The same push spread over a large area gives low pressure, while concentrated on a tiny area it gives high pressure: 10 pounds on one square inch is 10 psi, but on a hundredth of a square inch it is 1,000 psi. Read the other way, a given pressure on a larger area delivers more total force, the idea behind hydraulic pistons.

The center of gravity is the single point where an object's weight can be treated as acting. A dresser whose center of gravity sits behind its front feet will not tip over under its own weight; one whose center of gravity lies in front of them is unstable and will. Pulling out drawers, especially full ones, shifts that point forward, and a child's weight on an open drawer adds a turning force that can overpower the dresser's own steadying weight.

Density is mass per unit volume. An object less dense than the fluid around it rises and floats, while a denser one sinks. Ice is about 90 percent as dense as liquid water, because freezing locks the molecules into a more spread-out lattice, so an ice cube or iceberg floats with roughly a tenth above the waterline. The same rule runs a hydrometer, which floats higher in a denser liquid and lower in a less dense one.

Buoyancy is a body's tendency to rise or float in a fluid. Archimedes first worked out its size: the upward force equals the weight of the fluid the object pushes aside. An object heavier than the water it displaces sinks but seems lighter by that amount, which is why a rock feels heavier as you lift it out of a stream. A lighter object rises until it floats low enough to displace exactly its own weight.

A floating body displaces a weight of fluid equal to its own weight. Floating ice is therefore already pushing aside as much water as it weighs, and when it melts into water of that same weight, the level stays put. That is why melting ice cubes do not make a glass of ice water overflow, and why melting sea ice does not raise sea level directly. Ice that flows from land into the ocean is different: that added mass can raise sea level.

The pressure at a point in a liquid comes from the weight of liquid above it, so it rises in direct proportion to the depth and to the liquid's density, and at any depth it pushes equally in all directions. The container's width does not matter: 30 feet down a one-inch standpipe, the pressure is the same as 30 feet down in a large lake. Divers notice it on their eardrums within a few feet.

Pascal's law says pressure applied to a liquid sealed in a container is transmitted undiminished throughout it. Hydraulic systems, usually filled with oil, which barely changes volume under pressure, put this to work: a pump raises the oil's pressure, and the force on a piston equals that pressure times the piston's area. Oil at 1,500 pounds per square inch on a two-square-inch piston pushes with 3,000 pounds, and a larger piston pushes harder.

Viscosity measures how strongly a fluid resists flowing: the internal friction between its layers and against a solid surface. Thick oil has a high viscosity, while water's is low. For most liquids, viscosity falls as temperature rises: engine oil is thick when the engine is cold and noticeably thinner once it warms up. Gases have viscosity too, and the stickiness of air against an airplane's skin is a source of drag called skin friction.

Lift arises when a moving flow of air is turned by a solid surface: the air is deflected one way and the wing is pushed the opposite way. Adding up the pressures around the wing gives the same force, so the pressure-difference and flow-turning accounts are both correct. The popular equal-transit-time story, that air over the longer top must rejoin air from below, is wrong: the top flow moves far faster than that. Both surfaces turn the flow.

Drag grows with the square of speed and depends heavily on shape. Wind-tunnel tests comparing objects of equal frontal area give a flat plate facing the flow a drag coefficient of 1.28 and a typical airfoil about 0.045, a factor of almost 30. The back matters too: changing only the downstream shape can cut drag. A smooth surface also has less skin friction, the drag from air rubbing along it, than a rough one.

In a solid, molecular forces bind the molecules closely, so it keeps its own shape and volume. In a liquid those forces are weaker: it keeps its volume but takes the shape of its container. In a gas they are very weak, and it spreads to fill whatever holds it. Water shows all three, turning to ice below 0 degrees C and to vapor above 100 degrees C at standard pressure, and these are physical changes, not chemical ones.

Materials differ in how easily electric current flows through them. Metals offer very little resistance and are called conductors, while glass, plastic, porcelain, and dry wood slow or stop the flow and are called insulators. Metals pass heat along quickly too: a needle with one end in a flame soon carries the heat to the fingers holding it. Water is a twist: pure water conducts poorly, but dissolved salt makes it a conductor.

Glass is amorphous: on a large scale its atoms form a disordered network rather than a crystal's repeating pattern, yet it behaves like a solid. Amorphous silica, for example, is a glass below its glass transition temperature, about 1480 K at normal pressure, and a melt above it. By a common rule of thumb, an amorphous material counts as glass once its viscosity reaches about 10^12 pascal-seconds. Its atomic-level nature is not well understood.

A polymer forms when many small molecules are chemically joined into very large ones; the non-stick coating on a pan is one example, bonded to the pan at very high temperature. Glues show two families. A thermoplastic hardens by a physical change, such as a hot melt cooling down, while a thermosetting one hardens by a chemical reaction that cross-links its chains into a network that resists softening when heated.

Under the FTC's guides for green marketing, an item should not be marketed as recyclable unless an established program can collect or otherwise recover it for use in making another item. An unqualified claim needs recycling facilities available to at least 60 percent of consumers or communities where it is sold. Design matters too: shape or size can keep a recyclable material out of programs, and few programs can separate bonded layers.

An alloy combines a metal with other metals or elements, as cupronickel blends copper with nickel. Stainless steel, an alloy, resists corrosion because it grows a protective oxide film only about a dozen atoms thick. Brass and bronze are alloys of copper, and a niobium-titanium alloy is easy to draw into wire. Alloys can still fail chemically: in dealloying, brass can lose its zinc and leave behind a porous plug of copper.

An adhesive holds materials together by surface attachment. On porous materials such as wood, the main grip is probably mechanical interlocking: liquid glue flows into the pores, then anchors there as it solidifies. Molecular attractions such as hydrogen bonds add to the bond. Either way, glue must first wet the surface, spreading flat rather than beading. It then sets by drying, by cooling from a melt, or by a chemical cure.

A liquid spreads into a thin film only on a surface that attracts it strongly; where that pull is weak, it stays a rounded drop. Non-stick pan coatings are fluorinated polymers used for their resistance to water, oil, and grease. PTFE, a polymer of repeating CF2 units, has exceptionally low surface energy; in one test, water sat on untreated PTFE at a contact angle of 110 degrees. Related chemicals help fabrics resist stains.

The dew point is how cold the air would have to get, with its moisture unchanged, to reach 100 percent relative humidity. A windshield that cools below it collects dew, and air cooled further sheds water as fog. Relative humidity can mislead: 30 degree F air with a 30 degree dew point reads 100 percent, yet an 80 degree day with a 60 degree dew point, only 50 percent, feels far more humid.

Air that rises meets lower pressure, expands, and cools, about 5.5 degrees F for every 1,000 feet while it is still unsaturated. Once it cools to saturation, water vapor condenses onto cloud condensation nuclei, specks of dust, smoke, or sea salt, because water molecules alone are too small to start a droplet. That is why a cumulus cloud's underside tends to be flat: it marks the height where condensation begins.

Follow a snowflake falling from a winter cloud. If the whole column stays at or below freezing, snow lands. A thin warm layer only partly melts a flake, which refreezes as sleet; a deep warm layer melts it into a raindrop, and if the cold layer at the ground is too shallow to refreeze it, the drop freezes on contact as freezing rain. Hail is different: it grows in thunderstorm updrafts and falls once they can no longer hold its weight.

A lightning channel can reach about 54,000 degrees F, and it heats the surrounding air in a fraction of a second. Air heated that fast expands so violently that it compresses the air ahead of it into a shock wave, much like a sonic boom. Nearby you hear a sharp crack. Farther away, shock waves from every point along the bolt arrive stretched out and overlapping, and together they make the long rumble.

On a weather map, isobars are lines joining places with equal pressure. The pressure gradient force pushes air from high toward low pressure, and wind speed rises as the gradient steepens, so tightly packed isobars mark the windiest areas. Earth's rotation adds the Coriolis force, which deflects moving air rightward north of the equator and leftward south of it, and surface friction slows the wind and angles it across the isobars.

Fog forms when air near the ground reaches 100 percent humidity. Radiation fog forms on clear, calm nights as the ground radiates away the day's heat and chills the moist air above it. Advection fog is not caused by the ground cooling: warm, moist air drifts sideways over an already cold surface such as snow. Steam fog appears when cold air flows across warmer water, and the moist air above the water cools to saturation, rising like smoke.

Evaporation is liquid changing to gas, and it takes energy, which the sun, the air, the ground, or your own skin can supply. Water evaporates readily at its boiling point but keeps evaporating, much more slowly, even at its freezing point, which is how a puddle disappears on a cool day. Dry air helps because unsaturated air has room for more vapor, and the heat carried off is why wet skin feels cool after a swim.

Petrichor is the scent that rises when rain falls after a long dry spell, and it comes from the soil, not the raindrops: rewetting dry ground sets off a burst of volatile compounds, 58 of them in one study of Israeli soils. Geosmin, made mainly by soil bacteria called actinomycetes, is often identified with the smell, though that study measured little of it, and people can detect it in drinking water at a few billionths of a gram per liter.

Earth's axis is tilted about 23.5 degrees, so through the year each hemisphere takes a turn leaning toward the sun. The hemisphere tipped sunward gets a high sun angle, long days, and sunlight that crosses less atmosphere before reaching the ground. Distance cannot be the cause: Earth makes its closest approach to the sun in January, during Northern Hemisphere winter, and the yearly change in distance is too small to matter much.

The sun always lights half of the moon, giving it a day side and a night side. As the moon circles Earth, over a cycle of phases lasting about 29.5 days, you see different amounts of that lit half: almost none at new moon, when the side facing away from us is the sunlit one, and all of it at full moon. Earth's shadow plays no part in the phases. It touches the moon only during a lunar eclipse, which can happen only at full moon.

A solar eclipse needs a new moon and a lunar eclipse needs a full moon, yet both phases come every month while eclipses do not. The moon's orbit is tilted about 5 degrees from the plane of Earth's orbit, so at most new moons the moon slips just above or below the sun in our sky and its shadow falls past Earth. Only in two windows a year, called eclipse seasons and each about 35 days long, do the three line up.

The moon does rotate: one turn on its axis takes exactly as long as one trip around Earth, so the same face always points at us. This is called synchronous tidal locking, and every large moon in the solar system is locked to its planet the same way. Early on, Earth's gravity kept tugging at a bulge in the young moon, and the flexing drained energy until spin matched orbit. The far side is hidden, not dark; at new moon it is fully sunlit.

The moon pulls hardest on the side of Earth facing it and least on the far side, and that uneven pull squeezes the oceans into two bulges, one toward the moon and one away. As Earth turns, a coast passes through both, so most shorelines see two highs and two lows, with high to high taking a little over 12 hours. The sun's tidal force is about half the moon's; at new and full moon, when they line up, the two add up to the highest tides.

Near the equator, daylight stays within a few minutes of 12 hours all year. Moving toward a pole, the tilt stretches summer days and shrinks winter ones, only slightly in the tropics and dramatically at high latitudes. In South Dakota, for example, days run from about 15 hours near the June solstice to about 9 near the December one, and at the North Pole the sun stays up for about six months and then stays down for six.

Solar noon is the moment the sun crosses your meridian, the north-south line through the sky overhead. Clocks keep one time across a whole zone, so for each degree you sit west of the zone's central meridian the sun peaks about 4 minutes later; Omaha, 6 degrees west, runs 24 minutes behind. The equation of time, from Earth's tilt and oval orbit, shifts it by up to about 16 minutes over the year, and daylight saving time pushes it near 1 p.m.

Your eardrum separates the ear canal from the air-filled middle ear, and a narrow passage, the Eustachian tube, links the middle ear to the upper throat to keep its pressure steady. When cabin pressure shifts quickly on takeoff or landing, the tube can become flattened, so the eardrum bulges out or is pulled in, which hurts and muffles sound. Swallowing and chewing work muscles that open the tube so the pressures can match.

Piloerection, the scientific name for goosebumps, is the contraction of tiny arrector pili muscles in the skin, which stands hairs on end and raises bumps. Other animals use it to hold in heat and in threat or courtship displays. In people it is often called a useless leftover, yet in a study of eight volunteers, cold, touch and stirring videos triggered it about equally, and episodes tended to follow a drop in skin temperature.

Your hypothalamus, a brain region that works like a thermostat, compares temperature signals from the skin and from blood reaching the brain against a set point. In the cold, skin vessels narrow to keep blood near the core, and skeletal muscles shiver to make heat. In the heat you sweat, but sweat cools you only as it evaporates: sweat that does not evaporate costs body fluid while removing little heat, and humid air slows evaporation.

Delayed-onset muscle soreness, or DOMS, is the stiff ache that builds hours after unaccustomed hard exercise, especially eccentric work, where a muscle is forcibly lengthened, as in downhill running. It peaks one to three days later and fades within about a week. Lactic acid cannot explain that timing: its levels return to normal within an hour. A recent review cites several interacting causes, including inflammation and sensitized pain nerves.

A hiccup starts with a sudden, involuntary contraction of the diaphragm and the muscles between the ribs, pulling in a quick breath. The glottis in the voice box then snaps shut, and the cut-off rush of air makes the "hic." Nerves carry the trigger to a brainstem center and back to the diaphragm, a loop called a reflex arc. Big meals, alcohol and excitement can start a bout, and no benefit for adults is known.

Brain freeze, or cold-stimulus headache, is a short headache set off when something cold touches the palate or back of the throat; it clears within about 10 minutes once the cold is gone. The pain is typically felt across the forehead, though some feel it at the temples or behind the eyes. Its cause is debated: one idea is rapid narrowing and widening of blood vessels firing their pain sensors, another is direct stimulation of cold receptors.

Each semicircular canal in your inner ear is a fluid-filled loop crossed by the cupula, a gel-like flap that sits on sensory hair cells. When your head turns, the fluid moves and bends the flap. Spin at a steady speed and the flap slowly returns to center, so the sense of turning fades. Stop suddenly and the slowdown bends it the opposite way, signaling a spin in reverse, and the room can seem to rotate too.

Alcohol widens small blood vessels in the skin, a change called vasodilation, likely shifting warm blood from the core to the surface. A review of eight small lab studies, all of young men in heat, found alcohol lowered core temperature in three of the four that measured it; in one, drinkers felt hotter. A US hospital study also notes that alcohol intoxication is the most common cause of accidental hypothermia in otherwise healthy young people.

Belly rumbles and gurgles are thought to arise when gas and liquid mix during peristalsis, the squeezing waves of gut muscle that push food along, though in one ultrasound study many sounds did not line up with visible peristalsis nearby. Between meals a cleanup pattern, the migrating motor complex, sweeps the stomach and small intestine every 80 to 150 minutes, and in one person's long recordings, sound was much greater after a meal.

Over a day, gravity and activity load the discs between your vertebrae and squeeze fluid out, so your backbone shortens. Lying down at night takes that pressure off, and the discs draw fluid back in by osmosis, restoring their length. Biomechanics studies report adult height swinging by up to about 17 mm over 24 hours, and scans of 32 children having radiotherapy showed their lower spine about 1 mm shorter in the afternoon than in the morning.

During a warm soak, sympathetic nerves, part of the autonomic system that runs automatic body functions, make the small arteries in your fingertips narrow. With less blood filling the pads, the skin folds into wrinkles. Nerve studies show it: skin whose sympathetic nerves are cut or blocked does not wrinkle, even with the same soak. One study found wrinkled fingers grip wet objects with less force, a possible reason the response exists.

The photic sneeze reflex is a sneeze, or a nose prickle, when eyes used to dim light meet bright light, usually sunlight; it affects about one in four people. One idea is crosstalk, with strong optic-nerve signals spilling into the trigeminal nerve that supplies the nose lining, but none is confirmed. An EEG study of 10 photic sneezers found their visual cortex more excitable than controls', so the cortex seems involved, not just the brainstem.

A bruise is blood escaped from its vessels into tissue, where red cells soon burst and release hemoglobin, the protein inside them. Over days the patch goes from red to brown as the hemoglobin oxidizes, then to green and yellow, the colors of the pigments biliverdin and bilirubin; cells called macrophages make biliverdin by splitting heme. Colors vary so much that medical specialists judge a bruise's age correctly only about half the time.

Tears from glands above your eyes keep the eye's surface moist, and a complete blink, one that fully covers the cornea, the eye's clear front layer, renews and spreads that tear film before it breaks up. The spontaneous blink rate tracks the task: one study counted about 26 a minute in conversation, 17 in quiet gazing and 4.5 while reading. Screens are linked to more incomplete blinks, which leave the eye exposed.

The iris, the colored part of your eye, sets how much light enters through the pupil. Light makes its sphincter muscle constrict the pupil, and an opposing dilator muscle widens it again, each run by a different branch of the autonomic nervous system. The pupil can range from about 8 mm wide down to 1.5 to 2 mm, and in bright light it starts shrinking as soon as about 0.2 seconds after. Light shone into one eye constricts both pupils equally.

Damaged cells in an injured vessel wall send chemical signals that first narrow the vessel. Platelets, tiny cell fragments in blood, then stick to the wall and clump into a plug. They release molecules that switch on clotting factors, including fibrin, a sticky protein whose mesh holds the plug in place and traps red cells. The clot is later broken down; a research summary notes that pulling off a scab also removes new tissue growing beneath it.

Paresthesia is a tingling, prickling, burning or itching feeling, most often in the hands, arms, legs or feet. Steady pressure on a nerve can bring it on: long cross-legged sitting can squeeze the common peroneal nerve in the leg, leaving the top of your foot or the outside of your lower leg briefly numb or tingly until you shift. That discomfort prompts you to move, which matters because a nerve kept compressed long enough can be injured.

Tinnitus is hearing a sound with no outside source, often ringing, though some hear buzzing or roaring. Many notice it after a loud concert, game or job, and loud noise can cause hearing loss that seems to clear in 16 to 48 hours, though some damage may last. A leading theory: inner-ear damage changes nerve signals, and the auditory cortex, the brain's hearing area, generates the phantom sound. Human hair cells killed by noise do not regrow.

Sensing food by sight, smell and taste starts the cephalic phase, the first stage of digestion; saliva flow is among its earliest described responses, reported by Pavlov. He noted his dogs' stomach secretions could start when food was merely near or an attendant's hands smelled of it. He also showed salivation can be learned through conditioning; one study found cockroaches do it too, as an odor paired with sugar water came to raise salivation.

Breathing brings in oxygen and also clears carbon dioxide, a waste product that would otherwise make your blood more acidic. A control area in the brainstem's medulla sets breathing rate using chemoreceptors, cells that sense blood gases. Brainstem sensors respond to high carbon dioxide; sensors in the carotid bodies, small organs in arteries to the head, respond to high carbon dioxide and low oxygen. In exercise, both changes speed breathing.

In the retina, photoreceptor cells turn light into electrical signals, the raw input of the senses. Perception is the brain processing those signals to interpret the world, and one framework treats it as a blend of sensory evidence and prior knowledge. An illusion is a percept that differs from the physical stimulus. The steady world you see is one: receptors keep signaling motion as your eyes and head move, yet the scene looks still.

A just noticeable difference, or JND, is the smallest change in a stimulus you can detect. Weber's law makes it a fixed fraction of the starting intensity; early experiments found the detectable weight difference grew in step with the reference weight. The rule roughly holds for sight, hearing, pressure, smell and taste, but breaks down for faint stimuli such as dim light. So added weight obvious in a light bag may go unfelt in a heavy one.

Sensory adaptation is a drop in response after prolonged or repeated exposure to one stimulus, as sensory neurons adjust to recent input; it occurs in every sense. A lingering odor, for example, seems weaker over time. The fading can arise at the receptors or in the brain: in one study, a repeated odor was rated less intense though signals from the nose lining did not clearly drop. Without that filter the brain would be swamped.

Stare at a colored patch, then at gray, and a ghost patch appears in roughly the opposite hue: a yellow circle leaves a blue-purple one. Long viewing adapts the color-sensing system, making it less responsive to that color. Where that happens is disputed. Some textbooks credit opposing red-green and blue-yellow channels, but a 2025 study of many colors found afterimage colors fit a model of adaptation in the eye's cone cells instead.

The retina captures a flat image, so depth is rebuilt. Binocular disparity, the slight sideways offset between the two eyes' views, is one cue; motion parallax, where nearer objects seem to slide past faster, is another. A still photo offers only pictorial cues, such as parallel lines converging in the distance, one object partly hiding another, and far things looking hazy. A real scene seen with one eye loses surprisingly little apparent depth.

Top-down processing means your knowledge and expectations shape what you perceive, not your senses alone. Heavily degraded speech can sound meaningless to a listener with no idea what is coming, yet become intelligible right after they hear a clear version. In one set of experiments, printed words made degraded speech sound clearer when shown before the audio; the boost faded if the text came over about 0.12 seconds after the speech began.

Change blindness is failing to notice a salient change to a scene that happens across a brief disruption, such as a blank flash between two pictures, though everything is in view. In lab flicker tasks the two images alternate until the viewer finally spots the difference. It may matter on the road: in one simulator study, 16 drivers with normal vision missed about 6% of pedestrians who appeared while they looked elsewhere.

Your sense of taste reports at least five basic qualities: sweet, sour, bitter, salty and umami. Flavor blends those tastes with aroma, texture, temperature and sensations like the bite of chili. Chewing releases odors that travel up a channel from the roof of the throat to smell-sensing cells in the nose. When a cold blocks that channel, much of the flavor fades. Most people who think their taste is failing turn out to have a smell problem.

Microbes, often called germs, are too small to see without a microscope and live in air, soil, water, and on and inside your body. Most never make you sick. Gut bacteria help you digest food and absorb vitamins, and bacteria are used to make yogurt and cheese. The ones that cause infection are called pathogens, and of the vast variety of viruses on Earth, just a few can infect people.

A bacterium is a living organism made of a single cell, and antibiotics work by killing bacteria or making it hard for them to grow and multiply. A virus is a very tiny germ: genetic material, DNA or RNA, inside a protein coat. It invades a living cell and uses it to make copies of itself. Antibiotics do not work on viruses, including those behind colds and flu, even when mucus turns thick, yellow, or green.

Two families of bacteria matter in stored food. Spoilage bacteria give food off tastes, smells, and textures, and they can grow even in the refrigerator, but eating spoiled food usually would not make you sick. Pathogens, the kind behind foodborne illness, usually leave food looking and smelling normal, so a sniff test cannot tell you food is safe. The food's own enzymes also keep changing it, and freezing slows that work without stopping it.

Between 40 and 140 degrees F, a range often called the danger zone, bacteria grow fastest, doubling in as little as 20 minutes. Food that lingers there gives them time to reach harmful levels, so USDA guidance limits food to 2 hours out of refrigeration, or 1 hour above 90 degrees F. A big pot of soup cools slowly and spends a long time in that range, which is why leftovers go into shallow containers to chill fast.

Fermentation deliberately fosters microbes, such as lactic acid bacteria or yeasts, in a food. Their products, including lactic and acetic acid, carbon dioxide, ethanol, and bacteriocins (antimicrobial molecules some bacteria release), hold back spoilage and disease-causing organisms, so the food keeps longer. A starter culture adds wanted microbes in bulk to outcompete the rest, as in yogurt; kimchi relies on the food's own microbes.

Molds are microscopic, many-celled fungi. Their spores travel on air, water, or insects, and where one lands in warm, damp conditions it grows root threads into the food, a stalk above it, and spores at the tip, which give the fuzz its color. In a porous food like bread the threads can reach well below the surface, so the spot you see is only part of the mold. Dense hard cheese, by contrast, resists it.

Handwashing is mostly about removal. Surfactants in soap loosen dirt and microbes from skin, scrubbing adds friction, and a good lather forms tiny pockets called micelles that trap what comes loose. Rinsing under clean running water then washes it down the drain. Water temperature makes no difference to how many germs come off, and plain water rarely kills germs unless it is hot enough to scald. Hand sanitizer works differently, by killing germs.

Antibiotic resistance means the germs, not your body, can now defeat a drug. Under antibiotic pressure, resistant bacteria survive, multiply, and can pass resistance traits in their DNA to other germs, even ones never exposed to the drug. Every course carries that risk, and the drugs also kill helpful germs. Antibiotics do nothing against viruses like colds, so taking them for one cannot help yet can still cause side effects and resistance.

Drying and salt curing both pull water out of food; with too little moisture, bacteria cannot multiply and enzymes can barely act, which is why dry-cured country ham keeps at room temperature. Canning heats food in sealed containers to destroy microbes and inactivate enzymes, and the vacuum seal keeps new bacteria out. Freezing does not kill microbes but leaves them dormant until thawed. Molds tolerate salt and sugar better than most microbes.