I know you are, but what am I?
what bounces off me, sticks to you. Viva Peewee Herman
a character that was called Pee Wee Herman had a kid show called Pee Wee's Fun House. I think it was mainly an American kid's show but he spoke in a silly voice and would taunt other kid's by saying 'I know you are, but what am I. Its really just a kid thing, it doesn't have any other meaning except being an insult between 8 year old's. I think it's just something that's more common in the US.
No, I'm guessing that dlowan had not a clue about Peewee Herman.
I had no idea either. I think it is just an American thing.