Q: What do you call a bear with no teeth?
A: A gummy bear!

Explanation: The soft, red part that your teeth seem to come out of is called your gums.  When you lose a tooth your gums will probably bleed a little but it is nothing to worry about.

A gummy bear is a type of candy (also spelled gummi bear).  I like how they feel when I bite into them.

A bear that has no teeth probably still has gums.  This play on words makes him a gummy bear and makes this joke funny.

There is also a TV series called Gummi Bears.  Here is the first episode: