"Knock knock."
"Who's there?"
"9/11."
"9/11 who?"
"You said you'd never forget!"
"Knock knock."
"Who's there?"
"9/11."
"9/11 who?"
"You said you'd never forget!"
Listen here, smart guy. The joke is making fun of 9/11. The person says "9/11" for the knock-knock joke, and then when asked "9/11 who?", the joke says "You said you'd never forget!" It's funny because people DIED on 9/11. What are you, some kinda history denier or something?