Why Do Hot Peppers Cause Pain?

Reactions is taking a look into spicy science. That's right--the chemistry of capsaicin and why hot peppers cause pain. You have probably had the burning sensation of eating a hot jalapeno or other tear-inducing pepper. What causes this painful fire in your mouth? The short answer is capsaicin. But what exactly is capsaicin? How does it work? Why do people drink milk and not water?