Today I told my sis, "Knock knock."
She said, "Who's there?"
I said, "I Eat eat my mop."
She said, "I eat mop poo instead of who."
Listen up, you dummy! Your sister tried to be a smart aleck and messed up the punchline of the knock-knock joke. Instead of responding with "I eat mop who?", which would have completed the joke, she said "I eat mop poo instead of who".