a mouth sore that doesn't go away after two weeks can be cancerous. your dentist can look at them and tell you what they are.
but if the sore is like a white crater (ulcer), then it's just a canker sore. they hurt, but are essentially harmless (although because they are open sores, you need to avoid oral sex because germs can get in and out very easily).
And you can get blisters inside your mouth. If you eat really hot food and it burns your mouth, you can get a blister.
"Have you ever taken a moment to consider the alternate meaning of the word 'debugger?'" quoth Jack
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