In or out? That's the first question to ask when you're trying to figure out whether you have a canker sore or a cold sore. If it's inside your mouth, it's most likely a canker sore; outside, probably a cold sore.

The two may seem similar, but the similarity ends with the fact that both are connected with the mouth and both cause pain and discomfort. So knowing which one you have is the first step to knowing how it's caused and how to treat it.

Canker sores (or mouth ulcers) generally occur inside the mouth. They often are triggered by trauma such as biting your cheek, jabbing your gum with your toothbrush or even overzealous tooth cleaning. Cold sores are tiny, clear, fluid-filled blisters that form around the mouth and are caused by the herpes simplex virus living inside your nerve tissue. (It is usually not the same virus that causes genital herpes.)

Diligence is key when dealing with your oral health, says Academy of General Dentistry spokesperson Eric Z. Shapira, DDS, MAGD. "You need to be aware of any kind of lesion in the mouth or around the mouth, then report it to your dentist so he or she can play 'detective," says Dr. Shapira. "It is important to know what it is and what caused it before it can be treated properly."