Hot peppers trick your brain into thinking your mouth is on fire. But there's no real heat in a pepper. So, what's going on?

It's all about a chemical compound in peppers called capsaicin. Capsaicin binds to pain receptors on our nerves called TRPV1. 

Normally, it reacts to heat by sending warning signals to the brain. Capsaicin causes TRPV1 to send those same signals. So, you react as if there's something hot in your mouth. 

Your body tries to cool itself off. So, you start to sweat and your face turns red. At the same time, your eyes tear up and nose runs. This is your body's way of removing the "threat". 

After swallowing, the capsaicin binds to more receptors on its way down.