Q: Why did the students eat their homework?
A: Because the teacher said it was a piece of cake!

Explanation: “It’s a piece of cake” means that something is really easy.

Imagine a conversation between a teacher and a student: “What is 2 + 2 ?”

“Oh, that’s a piece of cake!  It’s 4!”

A piece of cake (you take an entire cake and cut a piece out and serve it on a plate) can also be just a piece of cake.  This joke is funny because it plays on the double meaning of “piece of cake.”