Knock knock. Who's there? Homer.

Homer who? Homer-ly, who could resist a classic like this? It's an *epic* pun, don't you think?

Knock knock. Who's there? Dictionary.

Dictionary who? I'm not defining the terms of this conversation.

Knock knock. Who's there? The door.

The door who? The door you just knocked on. It's not a person, it doesn't have a name.

Knock knock. Who's there? Cows go

Cows go who? No, cows go 'moo'