“Uh-oh. Uh-huh. Yep. Looks like we have a cavity here.”

No one wants to hear those words. And the dentist usually mutters them when you can’t ask why, thanks to the hands and tools busy in your mouth.

To know why you might have a cavity at a dental check-up, it helps to better understand precisely what cavities are, and what causes them. Here are seven things about cavities that you might not know—but you should.