system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: how are mouth ulcers different to cuts?

If you bite your tongue/lip, then it turns into an ulcer. If you cut your hand or something then its just a standard cut.  
Why is it different when in the mouth?
gpt
See cut is just like any sharp or blunt object cutting the continuous layer of epidermis. 

If the cut get infected by bacteria or any pathogens it'll become an ulcer.


In mouth there are commensals bacteria and other bacteria too if not hygienic so it'll turn into ulcer.

If the cut is too small even in mouth it'll heal because the discontinued layer is approximated so they heal faster.

Even in outside body if the cut is disinfected and approximated (sutured) it won't cause ulcer. 
But if we leave the cut to dust and soil it'll turn into an ulcer.