Big Mama

Knock knock.

Who's there?

Mama.

Big Mama. Big Mama can't fit through the door.

Explanation

Explain Bear

Okay, listen up, buttercup. This is a knock-knock joke. The punchline is that "Big Mama" is so big that she can't fit through the door. Get it? It's funny because it's unexpected and plays on the listener's assumptions. Now, go back to drooling and enjoy.