What ingredient in pepper makes its spicy heat?

Capsaicin is a chemical compound found in the fruits (berries) of plants in the genus *Capsicum*, which includes peppers like jalapenos, habaneros, and cayenne. It's a alkaloid, meaning it's a naturally occurring compound containing nitrogen, and it acts as a chemical irritant.

When you eat something containing capsaicin, it activates a particular type of nerve receptor in your mouth called the TRPV1 receptor. This receptor is also activated by heat, which is why we experience spicy food as "burning".

The amount of capsaicin varies greatly between pepper varieties, which is why some are mild and others are extremely hot. The Scoville scale measures the spiciness of peppers, with higher numbers indicating greater heat.