Knock knock. Who's there? Existential Crisis.

Existential Crisis who? I'm not sure, but the meaninglessness of this joke is a reflection of the void that lies at the heart of our existence.

Knock knock. Who's there? Olive.

Olive who? Olive this conversation, it's going nowhere!

Knock knock. Who's there? Mango.

Mango who? Mango-nificent to meet you!

Knock knock. Who's there? Cows go.

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