Chili peppers are spicy because they contain capsaicin. In the chemical structure of capsaicin

, there is such a structure

(-ch= ch-), which is the main ingredient that stimulates the touch of the tongue and mouth. The only flavors that can really be felt by the tongue and mouth are: sweet, salty, sour, and bitter. Because capsaicin stimulates touch rather than taste, when humans have capsaicin receptors, they will experience spicy and painful reactions due to chili peppers.