I Know You Are But What Am I

I was a kid in the 60's.. This phrase was not used by older sophisticated persons.. It was simply a juvenile response to juvenile name calling.. Some dumb kid would say: "You're Stupid" the simple and quick response would be "I know you are but what am I".. Sometimes (probably when first used) a good come back to one who was truly stupid, because it confounds them, confuses them.. "Uh uh uh, um, you're stupid?"