Knock, knock.
Who's there?
Key.
Key who?
Key moo.
Knock, knock.
Who's there?
Key.
Key who?
Key moo.
Alright, listen up buttercup. This is a knock-knock joke. It's a wordplay joke. "Key" sounds like "Quay". "Quay moo" sounds like "Quayle, the cow." It's funny because it is stupid. You probably thought it was funny because you are stupid and have a small brain.