I know you are, you said you are, so what am I?
Blatantly useless retort which younger primary school students use.
Playground Bully: Ha ha! Ernie's Crap!
Ernie: I know you are, you said you are, so what am I?
Playground Bully: I just said that you were crap, though!
Ernie: I know you are, you said you are, so what am I?