The Scientist: Kimberly Harms, DDS, is a spokesperson for the American Dental Association.The Answer: A canker sore is a little ulcer in the protective lining of your mouth. Unlike cold sores, which are caused by the herpes virus, canker sores are not the result of viral or bacterial infections and are not contagious. They’re auto-immune, which means that your body is basically attacking itself.Ulcer is a catch-all term for any hole in a bodily membrane. In the case of canker sores, the top layer of epithelial cells gets worn away, exposing the blood vessels and sensitive nerves underneath. Think of it like a blister with its outer dome dissolved.Though they know the immune system is responsible for creating these sores, doctors don’t understand what starts the reaction in the first place. They have their suspects. Munching into a sharp tortilla chip, the edges of broken teeth, braces and hard toothbrush bristles (always buy soft!) can cut the inside of your mouth, becoming the site of a painful sore. Acidic, citrusy, and spicy foods might be triggers, and if you already have a sore, these foods can make it worse. Theories suggest that canker sores might stem from allergic reactions to naturally-occurring oral bacteria, or sensitivity to the sulfate in toothpaste. Vitamin B12 deficiencies and gastrointestinal issues like Crohn’s and celiac disease could also play a role. And then there’s stress, which might cause canker sores by compromising your immune system.Canker sores are temporary and usually last a week or two. (If a sore sticks around longer than that, see a doctor; oral cancer looks similar.) Look for patterns—do you get canker sores after drinking lemonade or eating Indian food?—and try to cut out the trigger. Once you’ve got a sore, avoid hot, spicy or acidic foods, which can irritate it further and hurt like heck. Rinsing with warm salt water helps reduce swelling and promotes healing. In the meantime you can get an over-the-counter topical numbing agent at your drugstore to ease the pain.MORE ON ORAL HEALTH FROM YOUBEAUTY.COMThe Mouth-Health ConnectionEight Bad Brushing Habits That Harm Your TeethWhy You Have Bad Breath and How to Get Rid of ItOral Care Tips from Dr. Oz and Dr. Roizen