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Chili peppers don't actually increase the temperature of your mouth. Instead, they contain a compound called capsaicin, which activates pain receptors that normally respond to heat. Your brain interprets this signal as a burning sensation, even though nothing is actually hot. In response, your body may release endorphins, which is one reason many people enjoy eating spicy foods.
Drinking water doesn't remove the burning because capsaicin doesn't dissolve in water—milk usually works better because it contains casein, a protein that helps wash capsaicin away.
© National Institutes of Health