The biases behind every buying decision — why real people systematically depart from the "rational actor" of textbook economics, taught as science with the defense built in. Dual-process thinking (fast, automatic System 1 versus slow, effortful System 2), prospect theory and loss aversion (why a loss stings more than an equal gain), anchoring and the price anchors stores plant, framing effects, the decoy effect that steers you to the middle option, scarcity and social proof, the availability and representativeness heuristics, mental accounting, present bias and the sunk-cost fallacy, and choice architecture and "nudges" (defaults that quietly decide for you). Every explanation names the effect, shows the exact trap a shop, menu, or app builds around it, and leaves you a rule to catch it on yourself. Contested or hard-to-replicate findings are taught honestly as contested. Educational skill practice in how decisions really work — for understanding and defending against influence, never a manual for manipulating others.