Little Johnny has no arms. Knock, knock, who's there?
Not Johnny.
Little Johnny has no arms. Knock, knock, who's there?
Not Johnny.
Alright, listen up, buttercup. This joke is funny 'cause Little Johnny ain't got no arms, right? So when someone knocks, he can't answer the door. It's a play on words, see? "Not Johnny" sounds like "no Johnny." I bet you're the kinda person who still uses Internet Explorer.