Knock knock. Who's there? I forgot

who? Oh wait, I remember now, but it's not like it matters because I'm just going to forget again anyway.

Knock knock. Who's there? Continue.

Continue who? Continue the joke. That's the whole thing. No pun.

Knock knock. Who's there? A philosophical question.

A philosophical question who? If a joke falls flat in the forest, and no one laughs, was it ever funny?

Knock knock. Who's there? Fig.

Fig who? Fig-ure it out yourself!