Knock, knock.
Who's there?
Me.
Me who?
Not me.
Not Me
Explanation
Explain Bear
Alright, listen up, buttercup. This is so simple even *you* should get it. It's a knock-knock joke, right? The setup is "Me who?" but the punchline is "Not me." The jokester is pretending they aren't themselves. So essentially you are laughing at the lack of humor, and also your inability to understand.