Science Trivia curriculum

Each element has a symbol of one or two letters, and only the first letter is capitalized: C is carbon, Ca is calcium, and Cl is chlorine. Because a second capital letter starts a new symbol, Co is the element cobalt while CO is a compound of carbon and oxygen.

An element's atomic number is the number of protons in the nucleus of each of its atoms, and it alone decides which element an atom is: every atom with 6 protons is carbon, and every atom with 8 is oxygen. The periodic table lists the elements in order of increasing atomic number.

The vertical columns of the periodic table are groups and the horizontal rows are periods. Elements in the same group share similar chemical behavior, which is what makes the table a map of properties. IUPAC numbers the groups 1 to 18; older tables label them with Roman numerals and letters, such as IA and VIIA.

Group 1 below hydrogen holds the alkali metals: lithium, sodium, potassium, rubidium, cesium, and francium. They are soft, highly reactive metals that undergo similar chemical reactions. Hydrogen sits at the top of the column but is not counted among them.

Group 2 holds the alkaline earth metals: beryllium, magnesium, calcium, strontium, barium, and radium. They form similar compounds and are much harder than their neighbors in group 1, though still reactive.

The transition metals fill the broad central block of the table and include iron, copper, silver, and gold. As a class they are hard and dense and react less readily than the group 1 metals, which makes many of them useful structural materials.

Group 17, the halogens, runs from fluorine through chlorine, bromine, iodine, and astatine to tennessine. They react readily with metals: sodium and chlorine, for example, combine into sodium chloride, ordinary table salt.

Group 18 holds the noble gases: helium, neon, argon, krypton, xenon, radon, and oganesson. They are so unreactive that older tables call them the inert gases and number their column group 0.

Between the metals and the nonmetals sit the metalloids, elements with properties of both, such as silicon and arsenic. Some references call this third class the semi-metals; metals, nonmetals, and metalloids each occupy their own region of the table.

The two long rows printed below the main table are the lanthanoids and the actinoids, IUPAC's names for these series; older sources call them the lanthanide and actinide series. They are set apart mainly to keep the table from becoming too wide.

In 1869 Dmitri Mendeleev arranged the known elements by their properties and left gaps where no known element fit. The gaps predicted elements that had not yet been discovered, which is what made his table predictive rather than just a list.

The International Union of Pure and Applied Chemistry, IUPAC, decides when a claimed new element counts as discovered and approves its official name and symbol. Tennessine, element 117, is one of the elements named this way.

An element's standard atomic weight is not a fixed constant. It is reviewed periodically, and for some elements it is given as an interval rather than a single number, because the mix of isotopes in natural samples varies.

At ordinary room conditions most elements are solids; a few, including hydrogen, nitrogen, oxygen, fluorine, chlorine, and the noble gases, are gases; and only a handful are liquids. Mercury is the only metal that is a liquid at room temperature.

Some elements, such as gold, copper, and iron, have been known since ancient times and are listed as ancient rather than with a date. The heaviest elements are synthetic, made in laboratories, so each carries the year its discovery was reported.

The SI is built on seven base units: the meter for length, the kilogram for mass, the second for time, the ampere for electric current, the kelvin for temperature, the mole for amount of substance, and the candela for luminous intensity. Every other SI unit is built from these.

The SI unit of length is the meter, symbol m. It is defined through the speed of light in vacuum, one of the constants on which the SI rests: a meter is the distance light travels in a fixed fraction of a second.

The SI unit of mass is the kilogram, symbol kg, the only base unit whose name carries a prefix. It is now defined through a fixed value of the Planck constant instead of a physical object.

The SI unit of time is the second, symbol s. It is defined by a fixed frequency of the cesium atom, the radiation from one particular transition between its energy levels.

The SI unit of electric current is the ampere, symbol A. It is defined through the elementary charge, the charge on a single proton, so a current is ultimately a count of charges flowing past a point.

The SI unit of thermodynamic temperature is the kelvin, symbol K. It is fixed through the Boltzmann constant, and its scale starts at absolute zero, so 0 K is the coldest temperature possible.

The mole, symbol mol, is the SI unit of amount of substance. It is a fixed count of particles set by the Avogadro constant, so a mole of any substance contains the same number of atoms or molecules.

The candela, symbol cd, is the SI unit of luminous intensity, the brightness of a light source in a given direction as the human eye perceives it. It is defined through the luminous efficacy of a specified single-color source.

All seven SI base units now rest on seven fixed constants of nature: the cesium frequency, the speed of light, the Planck constant, the elementary charge, the Boltzmann constant, the Avogadro constant, and a luminous efficacy. A constant cannot drift or be damaged the way a physical standard can.

The speed of light in vacuum is one of the seven constants on which the SI rests, fixed exactly at 299,792,458 meters per second. Because the value is fixed, the meter is defined from it rather than the other way around.

Units for other quantities are built from the seven base units. Twenty-two derived units have special names, such as the joule for energy and the watt for power, and every other SI unit is a combination of these twenty-nine.

Twenty-four prefixes scale SI units by powers of ten, from quetta, 10^30, down to quecto, 10^-30. Each names a factor of the unit it is attached to, so the same prefixes work for meters, grams, seconds, or any other unit.

The prefix kilo multiplies a unit by 1,000 and the prefix milli divides it by 1,000, so a kilometer is 1,000 meters and a milliliter is a thousandth of a liter. They are the prefixes met most often in everyday measurement.

Four prefixes sit within a factor of a thousand of the unit: deci for one-tenth, centi for one-hundredth, deka for ten, and hecto for one hundred. Centi is the most familiar of them, as in the centimeter.

Beyond the everyday range the prefixes step by factors of a thousand: micro is one millionth, nano one billionth, giga one billion, and tera one trillion. A nanometer is a billionth of a meter, and a gigawatt is a billion watts.

NASA's planetary fact sheet lists eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Pluto appears in a separate column of its own rather than among the planets.

In order of distance from the Sun, the planets are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Earth orbits at about 150 million kilometers; Neptune is about 30 times farther out, at roughly 4.5 billion.

Jupiter is the largest planet. Its diameter, about 143,000 km, is roughly eleven times Earth's 12,756 km, and its mass is more than 300 times Earth's.

Venus, not Mercury, is the hottest planet, with a mean temperature of about 464 °C against Mercury's 167 °C. Venus has a crushing atmosphere, with a surface pressure about 92 times Earth's, that holds heat in; Mercury has almost none.

Venus is the planet closest to Earth in size: its diameter is 12,104 km against Earth's 12,756 km, and its mass is about 82 percent of Earth's. The likeness ends at the surface, which is far hotter and under far heavier air.

Read sizes from the diameter row, not from pictures, which are rarely to scale. The giants run Jupiter, Saturn, Uranus, then Neptune; among the rocky planets Earth is largest, then Venus, Mars, and Mercury, the smallest planet at 4,879 km.

Surface gravity differs from planet to planet. Jupiter's is 23.1 meters per second squared, more than twice Earth's 9.8, while Mars has about 3.7, a little over a third of Earth's. Weight changes with gravity; mass does not.

A planet's rotation period, the time to spin once, is not the same as its length of day, from one sunrise to the next. On Mercury one rotation takes about 59 Earth days, but a full day lasts about 176. Venus and Uranus rotate backward, in the retrograde direction.

Earth takes 365.2 days to orbit the Sun. The farther out a planet is, the longer its year: Mercury takes 88 days, Mars 687, Jupiter about 12 years, and Neptune about 164.

Obliquity is the tilt of a planet's spin axis relative to its orbit. Earth's is 23.4°, which gives it seasons. Uranus is tilted 97.8°, so it effectively rolls along its orbit on its side.

Earth has one natural satellite, the Moon, Mars has two small ones, and Mercury and Venus have none. The giant planets have many moons each, and their counts keep rising as small ones are found, so a fixed number is a poor fact to memorize.

All four giant planets, Jupiter, Saturn, Uranus, and Neptune, have ring systems. Saturn's are the most famous, but the fact sheet marks every giant as ringed and none of the rocky planets.

Mean surface temperatures run from Venus, about 464 °C, and Mercury, 167 °C, through Earth's 15 °C, down to about −200 °C at Neptune. Distance from the Sun sets the general trend; Venus's thick atmosphere is the exception.

The atmosphere is the air that surrounds Earth and is held to it by gravity. It is mostly nitrogen and oxygen, with smaller amounts of argon, carbon dioxide, and water vapor, and the weather happens in its lowest layer.

The troposphere is the lowest layer of the atmosphere, reaching from the ground up to the tropopause. Temperature generally falls with height through it, and it is where clouds, rain, and storms, the weather we feel, take place.

Above the troposphere lies the stratosphere, which extends up to the base of the mesosphere. It holds the ozone that absorbs much of the Sun's high-energy ultraviolet light before it reaches the surface.

Ozone is a form of oxygen whose molecule has three atoms, O3, rather than the two of ordinary oxygen. Near the ground it is a pollutant; high in the stratosphere it is essential, shielding the surface from ultraviolet radiation.

Evaporation is the change of a liquid into a vapor or gas; in weather it usually means liquid water becoming water vapor. It absorbs heat from its surroundings, which is why it has a cooling effect on whatever it leaves.

Condensation is the opposite of evaporation: a vapor becoming a liquid, or sometimes a solid. It is how clouds, fog, and dew form, and it releases back into the air the heat that evaporation absorbed.

A barometer is an instrument that measures atmospheric pressure. Weather systems are organized around centers of high and low pressure; an anticyclone, for example, is a large circulation of winds around a center of high pressure.

The Saffir-Simpson Hurricane Wind Scale rates hurricanes from Category 1 to Category 5 by their maximum sustained winds. Damage potential climbs steeply, roughly fourfold with each category, so a Category 4 storm is far more destructive than a Category 2.

The Fujita scale is the system for rating the intensity of tornadoes. It does for tornadoes what the Saffir-Simpson scale does for hurricanes, giving forecasters and the public a common shorthand for how strong a storm was.

Precipitation is water that falls from clouds to the ground as rain, drizzle, sleet, snow, or hail. It forms when water vapor condenses into droplets or ice crystals that grow heavy enough to fall.

Meteorology is the science of the atmosphere and its phenomena, above all the weather. Climatology, by contrast, studies climate, the long-run patterns and averages that weather adds up to over many years.

The greenhouse effect warms the lower atmosphere: sunlight passes in readily, but much of the longwave heat the surface radiates back out is absorbed on the way. The main greenhouse gases are water vapor, methane, carbon dioxide, and ozone.

Carbon dioxide, CO2, is a colorless, odorless gas and the fourth most abundant constituent of dry air, after nitrogen, oxygen, and argon. Plants take it in during photosynthesis, and burning fuels and breathing release it.

There is only one global ocean. Its waters are continuous, and the familiar ocean names are regions drawn on it for geographic and historical reasons, with boundaries that have changed over time.

Four ocean names are traditional: the Atlantic, the Pacific, the Indian, and the Arctic. Most countries, including the United States, now also recognize the Southern Ocean around Antarctica as a fifth.

Water covers about 71 percent of Earth's surface, so the planet is mostly ocean when seen from space. Land makes up the remaining 29 percent.

The Pacific is the largest and deepest of the ocean basins. Its rim is the Ring of Fire, a nearly continuous chain of ocean trenches, island arcs, and volcanic mountain ranges.

About 97 percent of Earth's water is in the ocean, which is why so small a share of the planet's water is fresh water available in lakes and rivers.

Tides are caused mainly by gravity. In 1687 Isaac Newton explained that ocean tides result from the gravitational pull of the Moon and the Sun on Earth's oceans, which raises and lowers the sea as Earth turns.

The Moon dominates the tides because it is so much closer to Earth than the Sun is. Even though the Sun is far more massive, its tide-generating force is only about half the Moon's.

A conductor is a material whose outer electrons are loosely bound and move freely, so electric current flows through it easily. Copper, silver, and gold are good conductors, and copper is the usual choice for wire.

The electron carries a negative charge and the proton an equal positive one. Unlike charges attract and like charges repel, a rule known as the first law of electrostatics.

An atom becomes an ion when it gains or loses electrons. Losing electrons leaves more protons than electrons, making a positive ion; gaining them makes a negative ion.

Every magnet has a north and a south pole. Like poles repel and unlike poles attract, the same pattern seen with electric charges. Earth itself has a magnetic field, generated by the spinning liquid outer core.

Ohm's law relates voltage, current, and resistance: E = I × R, so the current equals the voltage divided by the resistance. Doubling the voltage across a fixed resistance doubles the current, while doubling the resistance halves it.

The volt is the unit of electric potential difference, the push that drives current. One volt is the potential difference that makes one coulomb of charge do one joule of work, or equivalently the force that drives one ampere through one ohm.

The ampere is the unit of electric current, the rate at which charge flows. One volt drives a current of one ampere through a resistance of one ohm. Charge itself is counted in coulombs; one coulomb is about 6.28 × 10^18 electrons.

The ohm is the unit of resistance, the opposition a material offers to current. A resistance of one ohm lets one volt drive one ampere; a higher resistance means less current for the same voltage.

Faraday's law says the voltage induced in a coil depends on how many turns the coil has and how fast the magnetic flux through it changes. More turns, or faster motion of the conductor through the field, give a larger voltage.

A voltage is induced only while a conductor and a magnetic field move relative to each other. A wire held still in a steady field produces nothing; move the wire or change the field, and a voltage appears.

The atom is the smallest amount of matter that still has the properties of an element. Atoms are made of smaller particles, protons, neutrons, and electrons, but those particles on their own do not behave like any element.

An atom's nucleus holds protons and neutrons, together called nucleons. Their total is the mass number: an atom with 6 protons and 6 neutrons, carbon-12, has a mass number of 12, and the electrons add almost nothing to the mass.

Each element has its own atomic number because every atom of it has the same number of protons. Change the number of protons and the atom becomes a different element; change only the number of neutrons and it stays the same element.

Isotopes are atoms of the same element, and so of the same atomic number, with different numbers of neutrons. Carbon-12 and carbon-14 are both carbon; carbon-14 simply carries two more neutrons in its nucleus.

Most elements have a few stable isotopes and several unstable ones. An unstable, or radioactive, isotope eventually decays, giving off radiation as its nucleus changes.

The half-life of a radioactive substance is the time it takes for its activity to fall to half of its original value. After two half-lives a quarter remains, and after three an eighth, so the activity keeps halving at a steady pace.

In nuclear fission a heavy nucleus, such as uranium-235, absorbs a neutron and splits into two smaller nuclei of similar size, releasing energy and more neutrons. Those neutrons can split further nuclei, the chain reaction that powers a reactor.

The curie is a unit of radioactivity equal to 3.7 × 10^10 disintegrations per second, roughly the activity of one gram of radium-226. Another unit for the same quantity is the becquerel.

Kinetic energy is the energy an object has because it is moving. It grows with mass and, more steeply, with speed, rising with the square of speed: doubling an object's speed quadruples its kinetic energy.

Potential energy is energy stored by position. A raised weight has gravitational potential energy that turns into kinetic energy as it falls, and the higher it is lifted, the more energy it stores.

The first law of thermodynamics is the conservation of energy: energy can be neither created nor destroyed, only changed from one form to another. A power plant does not make energy; it converts the energy in its fuel into electricity and waste heat.

The second law of thermodynamics, in the form Rudolf Clausius gave in 1850, says no device working in a cycle can move heat from a colder body to a hotter one with no other effect. Heat flows from hot to cold on its own; a refrigerator needs work to reverse it.

Entropy measures how much of a system's energy is no longer available to do useful work. In real processes the total entropy increases, which is why no engine can turn all of its heat into work.

Four temperature scales are in common use. Fahrenheit and Celsius are relative scales, related by °F = 32 + (9/5)°C. Kelvin and Rankine are absolute scales that start at absolute zero: kelvins are about °C + 273, and degrees Rankine about °F + 460.

Absolute zero is the lowest temperature possible, the point at which a substance would have no thermal energy left to give up. It is 0 on the Kelvin and Rankine scales, about −273 °C, or about −460 °F.

At normal atmospheric pressure water freezes at 0 °C, 32 °F, or about 273 K, and boils at 100 °C, 212 °F, or about 373 K. On the Rankine scale the same two points are about 492 °R and 672 °R.

Sublimation is a solid turning directly into a vapor without first becoming a liquid, and the term also covers the reverse change, from vapor straight to solid. Either way, the liquid phase is skipped entirely.

Melting, vaporization, and condensation all move heat in or out at a constant temperature: while ice melts, added heat goes into changing its state rather than raising its temperature. Heat absorbed or released this way is called latent heat.

Heat moves in three basic ways: conduction through a material, convection carried by a moving fluid, and radiation sent out as electromagnetic waves. Most real heat flow combines them, as in a pot heating on a stove.

Conduction passes heat through a material by direct interaction between its neighboring atoms and molecules, without the material itself moving. A metal spoon left in hot soup warms along its handle by conduction.

Convection carries heat by the movement of a fluid itself: warmed fluid rises, cooler fluid sinks to replace it, and the circulation spreads the heat. It is how heat moves through boiling water, a heated room, and the atmosphere.

Thermal radiation is heat carried by electromagnetic waves given off by any warm body. Unlike conduction and convection it needs no medium, which is how the Sun's heat crosses the vacuum of space to reach Earth.

A force is a push or pull that tends to accelerate a body in the direction it is applied. Because its direction matters as much as its size, force is a vector quantity, like displacement, velocity, and acceleration.

What changes an object's motion is the net force, the sum of all the forces acting on it with their directions taken into account. Equal and opposite forces on one object cancel, leaving its velocity unchanged.

Newton's first law says an object at rest stays at rest, and an object in motion keeps moving in a straight line at constant speed, unless a net force acts on it. This tendency to resist changes in motion is called inertia.

Newton's second law says acceleration is proportional to the net force and inversely proportional to mass, F = ma, and it points in the direction of the force. The same push gives a lighter cart a larger acceleration.

Newton's third law says that when one body exerts a force on a second, the second exerts an equal and opposite force on the first. A rocket pushes its exhaust backward, and the exhaust pushes the rocket forward.

Weight is a force, the pull of gravity on an object's mass. Because gravity differs from place to place, an object's weight depends on where it is, while its mass does not.

Mass is the amount of material in an object and stays the same everywhere; weight is the gravitational force on that mass and changes with location. On the Moon an astronaut keeps the same mass but weighs about one sixth as much as on Earth.

Static friction holds a resting object in place, and kinetic friction resists an object that is already sliding. Each depends on the normal force pressing the surfaces together and on the nature of the surfaces, but not on how large the area of contact is.

Power is the rate at which energy is used or work is done, measured in watts, or joules per second. Two motors can do the same work, but the more powerful one does it in less time.

Momentum, mass times velocity, is conserved in every collision. Kinetic energy is conserved only in elastic collisions; in inelastic ones, such as two railroad cars that collide and couple, some of it becomes heat and sound.

Mitochondria are organelles in the cell's cytoplasm where most of the cell's energy is produced. They even carry their own small circular chromosome, mitochondrial DNA, separate from the DNA in the nucleus.

A ribosome is a small structure made of RNA and protein, and it is where the cell builds proteins. It reads the instructions carried by messenger RNA and links amino acids together in the order those instructions specify.

Proteins are large, complex molecules that do most of the work in cells. They are needed for the structure, function, and regulation of the body's tissues and organs, and each one is built from a chain of amino acids.

Amino acids are the building blocks of proteins. There are twenty kinds, and a protein is one or more long chains of them folded into a particular shape, with the order of the amino acids set by a gene.

Mitosis is the process in which a cell copies its chromosomes and separates them into two identical nuclei before it divides. The result is two cells genetically matching the original, which is how the body grows and replaces cells.

Meiosis is the special cell division that makes egg and sperm cells. It halves the chromosome number, so each gamete carries a single set, and fertilization restores the full two sets in the offspring.

Human body cells are diploid: they carry two sets of chromosomes, one inherited from each parent, 46 in all. Egg and sperm cells are haploid, carrying a single set of 23.

Stars shine by nuclear fusion: in their cores hydrogen nuclei are squeezed together into helium, releasing energy. That energy heats the star and pushes outward against the gravity that would otherwise make it collapse. Stars are mostly hydrogen, with some helium.

When a high-mass star can no longer sustain fusion in its core, the core collapses, rebounds, and sends a shock wave outward that blows the star apart in a supernova. The explosion scatters material that later becomes part of new stars and planets.

What survives a supernova is an extremely dense core, either a neutron star or a black hole. Lower-mass stars such as the Sun end differently, leaving behind a white dwarf.

A planetary nebula is the expanding cloud of gas and dust that a dying star like the Sun sheds as its outer layers blow away. Despite the name it has nothing to do with planets, and the core left behind becomes a white dwarf about the size of Earth.

The Milky Way is our home galaxy, a spiral whose disk of stars spans more than 100,000 light-years. Earth lies along one of its spiral arms, about halfway from the center, and the solar system takes about 240 million years to circle it once.

A black hole is matter packed so densely that gravity just beneath its event horizon is strong enough that nothing, not even light, can escape. Some form from the collapsed cores of very massive stars.

A black hole's event horizon is not a solid surface like Earth's or the Sun's, but a boundary that contains all the matter making up the black hole. Anything that crosses it, light included, cannot get back out.

A compound is a substance made of two or more elements chemically combined in a definite ratio. Water is always two hydrogen atoms bonded to one oxygen atom, and its properties are nothing like those of either element on its own.

A solution is a homogeneous mixture of two or more substances, uniform throughout. Salt fully dissolved in water is a solution; sand stirred into water is not, because the sand settles out.

The solute is the substance that dissolves in a solution, usually the one present in the smaller amount. In salt water the salt is the solute and the water is the solvent.

The solvent is the substance that does the dissolving, the medium the solute spreads through. In a solution of sugar in water, the water is the solvent and the sugar is the solute.

Oxidation is the loss of electrons by an atom or ion. It always happens together with reduction, because the electrons one substance loses are gained by another.

Reduction is the gain of electrons, the partner of oxidation in every oxidation-reduction reaction. A short way to keep them straight: oxidation is loss, and reduction is gain.

pH is the negative logarithm of the hydrogen-ion concentration. Because the scale is logarithmic, each whole step is a tenfold change: a solution at pH 4 has ten times the hydrogen-ion concentration of one at pH 5.

Solids keep both their shape and their volume; liquids keep their volume but take the shape of their container; gases keep neither, spreading to fill any container. Heating or cooling moves a substance between these states.

Water's chemical formula is H2O: two hydrogen atoms bonded to one oxygen atom, with a molecular weight of about 18. The formula is fixed, so every molecule of pure water has exactly that composition.

Ordinary table salt is sodium chloride, NaCl, made of sodium and chlorine in a one-to-one ratio. NIST lists its formula as ClNa, writing the elements in alphabetical order, but it is the same compound.

The pH scale runs from 0 to 14. A pH of 7 is neutral, where pure water sits; values below 7 are acidic and values above 7 are basic. Because the scale is logarithmic, a pH of 5 is ten times more acidic than a pH of 6.

DNA, short for deoxyribonucleic acid, is the molecule that carries the genetic information for the development and functioning of an organism. It is copied each time a cell divides, so each new cell receives the instructions.

DNA is shaped as a double helix: two strands wound around each other like a twisted ladder. The sides of the ladder are sugar-phosphate backbones, and the rungs are pairs of bases held together by hydrogen bonds.

DNA uses four bases: adenine, cytosine, guanine, and thymine, written A, C, G, and T. They pair in a fixed way, adenine with thymine and cytosine with guanine, so one strand determines the sequence of the other.

A gene is the basic unit of inheritance, a stretch of DNA that carries the instructions for a trait. Most genes code for proteins, and humans have about 20,000 protein-coding genes.

An allele is one of two or more versions of a DNA sequence at a given location. A person inherits one allele of each gene from each parent, and the two may be the same or different.

When two different alleles are inherited, the dominant one shows in the trait and the recessive one is masked. A recessive trait appears only when a person carries two copies of the recessive allele, one from each parent.

A trait is a specific characteristic of an organism, such as eye color or height. Traits are set by genes, by the environment, or by both, and they can be qualitative, like blood type, or quantitative, like height.

A chromosome is a threadlike structure of protein and a single DNA molecule that carries genetic information from cell to cell. Humans have 23 pairs, one chromosome of each pair from the mother and one from the father.

The genome is the entire set of DNA instructions in a cell. In humans it consists of 23 pairs of chromosomes in the nucleus plus a small chromosome inside the mitochondria.

The nucleus is the membrane-enclosed organelle that holds a cell's chromosomes. In humans it contains 23 pairs of chromosomes, while a small separate chromosome sits in the mitochondria outside it.

Sex in humans is determined by the X and Y chromosomes: females typically have two X chromosomes, and males one X and one Y. Because the father passes on either an X or a Y, his contribution sets the child's sex.

Genetics is the branch of biology that studies inheritance, how traits pass from parents to offspring through genes. Genomics, a related field, studies an organism's entire genome at once.

Gregor Mendel, a nineteenth-century Austrian monk, worked out the basic laws of inheritance by crossing pea plants and counting the traits in their offspring. His work founded the science of genetics.

Mendelian inheritance is the pattern by which a single gene's alleles pass from parents to offspring, with dominant alleles masking recessive ones. Two parents who each carry one recessive allele have a one-in-four chance, with each child, of a child showing the recessive trait.

A mutation is a change in a DNA sequence. Germline mutations, in egg or sperm cells, can be passed on to offspring; somatic mutations, in the other cells of the body, are not inherited.

The adult human skeleton has 206 bones. The axial skeleton accounts for 80 of them, and the appendicular skeleton, the arms and legs together with the shoulder and hip girdles that attach them, accounts for the other 126.

The heart's interior is divided into four chambers. The right and left atria are thin-walled chambers that receive blood returning in the veins; the right and left ventricles are thick-walled chambers that pump blood forcefully out of the heart.

The kidneys are the main organs of the urinary system. They filter the blood, remove wastes, and excrete those wastes in urine; the rest of the system consists of structures that carry the urine out of the body.

The pancreas works in both digestion and hormone control. Cells in its islets release the hormone insulin when the glucose level in the blood is high and glucagon when it is low, the pair of hormones that keeps blood sugar in range.

The endocrine system works through hormones, chemical messengers that influence growth, development, and metabolism. Its effects come on more slowly than the nervous system's, over minutes, hours, or weeks rather than seconds, and they last longer.

The central nervous system is the brain and the spinal cord. The brain is protected by the skull and the spinal cord by the vertebrae, and the nerves outside them form the peripheral nervous system.

The skin is the body's largest organ. Covering the entire outside of the body, it is about 2 mm thick and weighs roughly six pounds, and it guards against injury, heat, light, and infection. Its outer layer, the epidermis, holds the cells that make melanin.

Earth has three main layers, often compared to a boiled egg: a thin, rigid crust; a thick mantle of hot, dense, semi-solid rock; and a central core. The crust is only about 5 km thick under the oceans and averages about 30 km under the continents.

The mantle is by far the thickest layer, about 2,900 km from the base of the crust to the core. The liquid outer core is about 2,200 km thick and the solid inner core about 1,250 km, while the crust is only tens of kilometers at most.

Earth's core has two parts, a liquid outer core and a solid inner core. As Earth rotates, the liquid outer core spins, and that motion generates the planet's magnetic field.

Plate tectonics is the theory that Earth's outermost layer is broken into large plates that move slowly over the layer beneath. Tectonics comes from a Greek root meaning to build, so the name describes how Earth's surface is built of plates.

In 1912 the German meteorologist Alfred Wegener proposed continental drift: that about 200 million years ago a supercontinent, Pangaea, began to split apart into the continents of today. Plate tectonics later explained how the continents move.

Most earthquakes and volcanic activity happen in narrow zones along plate boundaries, where plates pull apart, collide, or slide past one another. The boundary types are divergent, convergent, and transform, plus broad boundary zones where the edges are not sharply defined.

Igneous rocks form when hot, molten rock crystallizes and solidifies; the name comes from the Latin word for fire. Rock that cools slowly underground has a coarse texture, as in granite, while lava that cools quickly at the surface forms rocks such as basalt and obsidian.

Molten rock beneath the surface is called magma. When it erupts at volcanoes or oozes from fissures, the same molten rock is called lava, and it cools and solidifies almost at once in the comparatively cool air.

Sedimentary rocks form from deposits that collect at Earth's surface, pieces of older rock or of once-living things. Buried deeply, the sediment is compacted and cemented into rock, which often shows distinct layers; sandstone, shale, and limestone are common examples.

Metamorphic rocks are rocks changed by high heat, high pressure, hot mineral-rich fluids, or a combination of these, conditions found deep in the Earth or where plates collide. Common metamorphic rocks include schist, gneiss, quartzite, and marble.

A seismograph detects and records earthquakes. Its seismometer, a mass on a spring or a pendulum, stays nearly still while the ground and the instrument move around it, and the record of that relative motion is a seismogram.

Charles Richter developed his logarithmic magnitude scale in the 1930s to measure earthquakes in southern California, and it became the first widely known measure of earthquake size. Seismologists now rely on the moment magnitude scale, which works for large and distant earthquakes too.

The Mohs scale ranks minerals by hardness, their resistance to scratching, from talc at 1, the softest, to diamond at 10, the hardest. A harder material scratches a softer one, so a steel nail, about 6.5, scratches calcite, a 3, but not quartz, a 7.

Fossils are evidence of ancient life, the preserved remains and traces, such as footprints, of organisms that lived long ago. Paleontology is the science that studies them; vertebrate paleontology, for example, studies ancient animals with backbones.

Earth has seasons because its axis is tilted. Through the year first one hemisphere and then the other leans toward the Sun and receives its most direct rays, which brings that hemisphere summer. Distance is not the cause: Earth is closest to the Sun in January.

A solar eclipse happens when the Moon gets in the way of the Sun's light and casts its shadow on Earth, so the day goes dark for a few minutes. The shadow is small, so only places along its narrow path see a total eclipse.

A lunar eclipse happens when Earth gets between the Sun and the Moon, so Earth's shadow covers the full Moon. It can be seen from the whole nighttime side of Earth, and the Moon often looks reddish because Earth's atmosphere bends some sunlight onto it.

A meteoroid is a small rock in space. The streak of light it makes burning up in the atmosphere is a meteor, often called a shooting star, and if a piece survives the trip and reaches the ground it is called a meteorite.

Sunlight contains all colors. The gases and particles in the air scatter it in every direction, and blue light, which travels as shorter, smaller waves, is scattered more than the others, so blue reaches our eyes from all over the sky.

The Big Bang theory holds that the universe began in an extremely hot, dense state and has been expanding and cooling ever since. In the 1920s Edwin Hubble found the first observational evidence that space is expanding, which means the universe was once smaller.

Visible light is the narrow band of wavelengths the eye can see. A prism spreads it into the colors of the rainbow because each color has a different wavelength, from violet, the shortest at about 380 nanometers, to red, the longest at about 700.

Reflection is light striking an object and bouncing off it. Very smooth surfaces such as mirrors reflect almost all of the light that hits them, which is why they can show a clear image.

Refraction is the change in direction of light as it passes from one medium into another, because light travels at different speeds in different materials, slower in air than in a vacuum and slower still in water. The change in speed at the boundary is what bends the light.

Diffraction is the bending and spreading of waves around an obstacle or the edge of an opening. It is most noticeable when the obstacle is about the same size as the wave's own wavelength.

Light behaves both as a wave and as a stream of particles. The particles, photons, are discrete packets of energy that carry momentum, have no mass, and travel at the speed of light. Frequency, wavelength, and energy are linked, so knowing one gives the other two.

Light travels through a vacuum at exactly 299,792,458 meters per second, about 300,000 kilometers each second. The value is exact because the meter is now defined from it. At that speed sunlight takes a little over eight minutes to reach Earth.

When a sound source moves, its waves bunch up ahead of it, shortening the wavelength, and spread out behind it, lengthening it. Shorter wavelengths sound higher, so the pitch you hear changes as the source passes by. That change in pitch is the Doppler effect.

Sound is a pattern of small pressure changes passed along through a medium such as air. Its speed depends only on the state of that medium, not on the frequency, the wavelength, or any other property of the source that makes it.

Einstein's equation E = mc² says that energy equals mass times the speed of light squared, so mass and energy can transform into each other. Because the speed of light is so large, a tiny amount of mass corresponds to a huge amount of energy, as in the fusion that powers the Sun.

In special relativity the speed of light is the same everywhere, and nothing with mass can reach it. As an object approaches light speed it needs ever more energy to go faster, and reaching the speed of light itself would take an infinite amount.

Antimatter is made of antiparticles, partners of ordinary particles that carry the opposite charge. The positron is the antimatter partner of the electron, and even an ordinary banana releases one now and then, about every 75 minutes.

Producers are organisms that make their own food. Green plants are the classic example: they capture the energy of sunlight through photosynthesis, so they form the base of the food chain, and other creatures depend on them directly or indirectly.

Consumers get their energy by eating other living things. Herbivores eat only plants, carnivores eat only meat, and omnivores eat both plants and meat. Herbivores are the primary consumers, eating straight from the base of the food chain.

Decomposers are organisms that consume dead plants and animals. By breaking down dead matter they return its nutrients to the soil, where producers can use them again, closing the loop of a food chain.

Chlorophyll is the substance that colors plants green, and it is what lets them use the energy of sunlight to turn carbon dioxide and water into glucose and oxygen. That process is photosynthesis.

Transpiration is the release of water vapor from a plant's leaves after its roots take up liquid water from the soil. Together with evaporation from land and water surfaces it moves water into the atmosphere, a combined process called evapotranspiration.

Vertebrates are animals with a backbone, a column of vertebrae: fish, amphibians, reptiles, birds, and mammals. Animals without a backbone are invertebrates, and they are far more common than vertebrates.

Mammals are vertebrates with three defining features: mammary glands that produce milk to nourish their young, hair or fur, and warm-blooded bodies. Together those features characterize the group today.

Amphibians, the group that includes frogs, toads, salamanders, newts, and caecilians, are semi-aquatic: they can live both in water and on land. They absorb water through their skin, and some salamanders have no lungs at all, breathing through their skin and mouth.

Reptiles, the group that includes turtles, crocodiles, lizards, and snakes, are vertebrates with scales and at least one lung. Their amniotic eggs let them lay eggs on land rather than in water.

Reptiles and amphibians are ectotherms, often called cold-blooded: they cannot regulate their body heat internally, so they warm up by basking in the sun and cool off in the shade. In return they can live on far less energy than birds or mammals of similar size.