Take a bite of warm, gooey pepperoni pizza, and your digestive system kicks into action. Acids tumble into the stomach, ready to break down your meal. In many people, a faulty valve-like structure lets these acids run amok, triggering a burning sensation in the chest. Heartburn is an extremely common symptom. More than 60 million Americans have heartburn at least once a month. The following slides examine where heartburn starts, and how it can be stopped.