Q: Why did the Easter egg hide?
A: Because he was a little chicken!
Explanation: An egg will soon become a chicken so you could say that an egg is a little chicken, a small chicken (OK, I know that an egg is not a chicken, not yet anyway, but that idea helps make this joke funny).
“To be chicken” means to be scared: He’s too chicken to go down the slide = he’s too scared or too afraid to go down the slide. Sometimes people hide when they are scared like in this scene from the movie Jurassic Park.
So, when the egg hides it is a little chicken, both a scared chicken and and a small chicken!
Take a look at these Easter Eggs: