Knock knock.
Who's there?
Steven.
Can't you read? It says, "No Hawking."
No Hawking
Explanation
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.