Q: What do racers eat?
A: Fast food!

Explanation:  A racer is a person who races, such as a runner or a race car driver.  The whole point of a race is to go fast in order to win.  “Fast food” is food that you can get at a restaurant like McDonalds: you order the food and they give it to you right away.  So if you want to go fast you need to eat food that will give you the energy to go fast, or “fast food.”  That’s what makes this joke funny!  Of course fast food will not make you fast, fat perhaps, but not fast 🙂 .