I've got one doozy of a canker sore right now located at the most inconveniant spot just inside my lower lip where everything that I eat or drink makes direct contact with it. I don't get them as often as I used to, but when I do get one they freakin hurt like hell! I usually dab it directly with Peroxide with a q-tip to keep it clean until it heals, or squeeze it between my fingers until blood seeps to the surface relieving the pressure in it and bringing relief. Healing takes anywhere between a few days to a couple of weeks in the most stubborn cases, this one's been with me since last week and it's about the diameter of a pencil eraser. What causes these things? Why do they feel cold when you eat something hot, body heat escaping through it I suppose? And what exactly is it, an exposed nerve? It certaintly feels like that, sometimes it hurts so bad that it feels as if I have a toothache even though I don't, because that whole area of my face aches. I hate them, we need a cure pronto!