Knock knock. Who's there? No one.

No one who? Exactly.

Knock knock. Who's there? The.

The who? The door, silly! You just opened it.

Knock knock. Who's there? Nimbus.

Nimbus who? Nimbus sure I'm at the right door, the forecast said I'd find you here!

Knock knock. Who's there? Nobody.

Nobody who? Exactly. Why are you talking to the door?