Knock Knock / Who’s There? / Ahmed

Knock Knock
Who’s There?
Ahmed
Ahmed who?
Ahmed the payphone trying to call home. All of my change I spent on you.

This is a knock-knock joke. There are two critical elements in this joke that contribute to the humor. The first is the pun. The word Ahmed sounds sort of like "I'm at", which is not actually very funny at all because it is a terrible pun.

The other funny part is the fact that the punchline is the lyrics of a song - Payphone by Maroon 5. It's not particularly funny, but that is knock-knock jokes in a nutshell.