Why do spicy foods burn your mouth?

The heat of chilli peppers comes from capsaicin, a molecule that latches onto a receptor called TRPV1, the same receptor that normally detects genuinely hot temperatures and physical burning. When capsaicin activates TRPV1, your nervous system sends the brain the same signals it would for actual heat, so you feel a burning sensation and may sweat, tear up, or flush, even though your tissues are not truly being harmed. The intensity of a pepper is measured on the Scoville scale, which ranks peppers by how much capsaicin they contain — from mild bell peppers at zero to super-hot varieties exceeding two million units. Because capsaicin is oil-based, water does little to wash it away, but dairy products contain casein, a protein that helps break up and remove capsaicin, easing the burn.