Q: Why shouldn’t you tell an egg a joke?
A: Because it might crack up!

Explanation:  To crack up means to start laughing (sometimes it also means to to go crazy).  When you break an egg you first crack it open.

If an egg cracks up does it start laughing or does it just crack open?  This joke is funny because of the double meaning of the word ‘crack.’

More with egg: You can also be caught with egg on your face, be a good egg, and here are many more egg-spressions (expressions).

Learn an interesting way to crack an egg: