What red and bad for your teeth?

Q: What red and bad for your teeth?
A: A brick.

This is a riddle that is also a joke. The humor in this joke is that the listener assumes that the jokester is asking "what red thing is likely to cause cavities if you eat it", when the jokester is instead asking "what red thing would not be good to apply forcefully to your teeth."

The punchline disabuses the listener of this misconception, and that is where the humor comes in.