Knock knock. Who's there? Continue.

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

Knock knock. Who's there? Silence.

Knock knock. Who's there? Documentation.

who? This joke is still in beta and the punchline has not been implemented yet. Please try again later.

Knock knock. Who's there? (Knock knock.)

(Knock knock) who? Oh, for goodness sake, you just started the joke again! This is an endless loop!