A canker sore is a small, painful ulcer that appears inside your mouth, usually on the inside of your cheeks, lips, or tongue. Unlike a cut or bite that heals quickly, a canker sore develops on its own and creates a shallow, often round crater with a white or yellowish center and a red border.

These sores hurt more than you'd expect for something so small. Eating salty or acidic foods like potato chips or oranges can make them sting intensely. Even talking or brushing your teeth near one can be uncomfortable. They typically last one to two weeks before healing completely on their own.

Doctors aren't entirely sure what causes canker sores, though stress, mouth injuries from braces or aggressive tooth brushing, and certain foods seem to trigger them in some people. They're not contagious, meaning you can't catch one from someone else or spread yours to others.

Don't confuse canker sores with cold sores, which appear outside the mouth on your lips and are caused by a virus. If you get a canker sore, rinsing with warm salt water and avoiding foods that irritate it can help, but mostly you just need patience while it heals.