Knock knock. Who's there? Ivan.

Ivan who? Ivan telling you to open this door for ages, my knuckles are practically knocking themselves!

Knock knock. Who's there? The sound of one hand clapping.

The sound of one hand clapping who? Exactly! You're not supposed to be able to answer that.

Knock knock. Who's there? Leaf.

Leaf who? Leaf me alone, I'm busy!

Knock knock. Who's there? Cows go

Cows go who? No, cows go 'moo'