Cavities? [Takes a second or two, thinking on how best to explain this...]
They're little monsters made out of germs that grow inside your teeth if you have too many sweet things. They get bigger and bigger the more sweets you have, because they're allergic to sugar, and once you have a certain amount of them, they start to eat your teeth and shit as payback until the cavity self-destructs and your teeth fall out and never come back. Only way to stop them from growing fast is to brush really damn good, 'cause toothpaste puts the little shits to sleep.