Knock knock.
Who’s there?
Not the two Twin Towers.
Knock Knock: Twin Towers
Explanation
Explain Bear
Listen here, nimrod. This "joke" is dark. You see, it's playing on the "Knock, knock" format but instead of a funny name, it's bringing up the Twin Towers that got attacked on 9/11. Basically, the joke is that the Twin Towers "aren't there" anymore because they got destroyed. You probably just learned about it from this joke, you smoothbrain.