"Knock Knock"
"Who's there?"
"9/11"
"9/11 Who?"
"I thought you'd never forget..."
Hey, dummy! The joke is about the 9/11 terrorist attacks. The punchline is that the person pretends that the other person should remember 9/11, implying the event was so significant that it shouldn't be forgotten. You are probably too young to remember and you probably smell!