Knock knock.
Who's there?
Steven.
Can't you read? It says, "No Hawking."
No Hawking
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Explanation
Experiment
Explain Bear
Alright listen up, genius. The joke is a play on words. It uses the common "Knock knock" format. The punchline twists "Stephen" into "No Hawking", as in, "No soliciting allowed" or something like that. Now go back to drooling on yourself.