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!