Why does spicy food burn?

Chili heat is not a taste. Capsaicin, the active compound in chili peppers, binds to TRPV1, a receptor on sensory nerve endings that normally responds to temperatures above about 43 degrees Celsius and to acid. When capsaicin activates it, the nerve sends the same signal it would send if the tissue were genuinely being burned, and the brain responds accordingly with pain, sweating, flushing, and a runny nose. Nothing is actually being damaged at ordinary culinary levels, which is why the sensation fades without injury. The discovery of TRPV1 and related receptors earned a share of the 2021 Nobel Prize in Physiology or Medicine.

The remedies follow from chemistry. Capsaicin is fat-soluble and barely soluble in water, so drinking water spreads it around the mouth without removing it. Dairy works because casein, a protein in milk, helps strip capsaicin from the receptors, and the fat content dissolves it, which is why full-fat milk, yogurt, and lassi outperform skim. Fat in general helps, so oil or a fatty food works. Alcohol dissolves capsaicin but at drinkable concentrations is not very effective, and beer is mostly water. Sugar and starch provide some relief. Time works too, since the binding is reversible.

Two further points. Heat is measured in Scoville Heat Units, originally by a dilution taste test and now by chemical analysis converted to the traditional scale, running from bell peppers at zero through jalapenos in the thousands to the hottest cultivated varieties above two million. Tolerance is genuinely trainable, since repeated exposure depletes the neurotransmitter substance P and desensitizes the receptors, which is why people who eat spicy food regularly experience it less intensely. And birds are immune to capsaicin because their version of the receptor does not bind it, which appears to be the point evolutionarily: birds disperse chili seeds intact while mammals grind them up, so the plant evolved a compound that deters one and not the other.