Q: What is a plumber’s favorite food?
A: A leek!

Explanation: A plumber is a person who fixes water pipes if they are clogged or are dripping; that is, if they have a leak.

A leek (two e’s) is a vegetable a lot like a green onion.  A leak (one e, one a) is what happens to the water pipes when water drips out.

This joke plays with the word leek/ leak and that is what makes it funny.