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."
Mop Knock Knock Antics With Sis Humorously Derails Joke Structure
Explanation
Explain Bear
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".