Knock knock.
Who's there?
9/11.
9/11 who?
You said that you would never forget!
Knock Knock 9/11
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Explanation
This is a dark and morbid play on words using the 9/11 attacks. The setup is the classic "knock-knock" format, but instead of a name the response is "9/11", which is not a name but the date of the terror attacks. The punchline uses the common phrase "never forget" which is associated with that event.