Q: Why do bananas have to put on sunscreen before they go to the beach?
A: Because they might peel!

Explanation:  A banana peel is the outside skin of the banana.  Your skin can peel if you are in the sun and you get a sun burn.  In order to avoid a sunburn you need to use sunscreen.

This joke is funny because of the double meaning of the word peel (the outer skin of a banana and what your skin does when it gets burned.  Sunburns, though, are not funny- use sunscreen!