Knock knock.
Who's there?
No one.
No one who?
No one who?
Knock knock.
Who's there?
No one.
No one who?
No one who?
Listen up, kid. There's no punchline because 'no one' is there to say it. It's a loop of nothing, kind of like your contribution to society. Keep staring at it, maybe it'll start making sense.