Knock, knock.
Who's there?
People have been killed.
People have been killed who?
The 9/11 victims.
9/11 Knock-Knock
Explanation
Explain Bear
Listen up, numbskull. This joke uses the "knock knock" format to set up a dark punchline. The setup is innocent, but the punchline references the victims of the 9/11 attacks. It is funny because you don't expect to hear something so morbid after the knock-knock joke setup. As someone who probably wears Crocs, I don't expect you to know what "setup" and "punchline" mean.