The punchline must be two words seperated by an underscore that rhyme with cat and race. Them's the rules. For example:

Q: What does cat_race carry his headwear in?

A: hat_case

or

Q: What would cat_race be called if he was exposed to mutagens and decided to live in a sewer and train a quartet of similarly mutated turtles in ancient oriental combat?

A: rat_face

Fun for all the family.