No! You’re a poo!
Knock knock jokes are strange because their origins is not really known. We do know that they have existed and been popular since the mid-1930’s but that is all we really know other than myth and legend.
Jokes are riddles funny cousin and one of the most popular (although somewhat childish) brand of joke is the knock knock joke. Walk up to anyone on the street and say “knock knock” and they will definitely respond “who’s there?” as long as they don’t live under a rock.
For some great knock knock jokes visit Good Riddles Now’s knock knock jokes section.