What Makes Certain Foods Spicy But Not Sour?

Imagine your tongue has two different messengers. One messenger runs around looking for heat like a fire alarm. When you eat a chili pepper, it trips an alarm in your mouth that says 'It is hot!' even though the food isn't actually burning your skin. This happens because of a special chemical called capsaicin.

The other messenger looks for sourness. When you bite into a lemon, this messenger runs to tell your brain about the acid. It feels like a sharp pinch or tingle, not fire.

The hot messenger connects to nerves that usually feel warm temperatures. The sour messenger connects to taste buds that detect acids. This is why a chili pepper can make you sweat while eating a lemon just makes your mouth pucker. Your brain interprets the signals differently based on which messenger sends the message.