Knock, knock.
Who's there?
I did.
I did who?
You did a poo.
Alright listen up, dum dum. This joke is funny 'cause it's a knock-knock joke, right? But instead of a normal name, the dude says "I did". So when you ask "I did who?", it sounds like "You did a poo!" which is super immature. Bet you fart a lot, don't you?