Knock knock. Who's there? Olive.

Olive who? Olive your jokes are so funny!

Knock knock. Who's there? Dew.

Dew who? Dew-n't worry, it's just a drop in.

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? Cows go.

Cows go who? No, cows go 'moo'.