Knock knock. Who's there? Otto.

Otto who? Otto know by now that Otto-matically, I'd bring an out-of-this-world pun like this right to your door!

Knock knock. Who's there? Fig.

Fig who? Fig-ure it out yourself, I'm busy!

Knock knock. Who's there? Existential Crisis.

Existential Crisis who? I'm not sure, I've been questioning the meaning of my own existence and the nature of identity, so the concept of a 'who' is somewhat fluid to me.

Knock knock. Who's there? Brew.

Brew who? Tea-rific to meet you.