I know you are but what am I is the ultimate comeback for kids below the third grade. When a group of kids (mostly males) get together with minimum supervision, they start to pick on each other (in an Alpha Dog way). Scathing soul-shattering insults will be hurled back and forth like this:

First kid: "You're a doo-doo head."

Second kid: "You wet the bed."

First kid: "You kissed your sister."

Second kid: "You're a gorilla's butt." (major laughter from surrounding crowd... major score!)

First kid: "I know you are, but what am I?"

Now, no matter what the second kid says, the speaker of the ultimate comeback will just deflect any insult right back to the source. After four or five futile attempts, the second kid gives up in a huff, mentions "That's so lame!", then wanders off to throw dirt at the icky girls.

There actually is a comeback to the almighty "I know you are, but what am I?"

"It takes one to know one."

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This was demonstrated in the 'lemon tree episode' of the Simpsons.

Bart and co. gather around the Springfield lemon tree to bask in the citrus-y goodness. A few metres away, just over the Springfield/Shelbyville border, some hoodlums gather and the insults fly:

Shelbyville Kid: You're a garbage man.
Bart: I know you are, but what am I?
Shelbyville Kid: A garbage man.
Bart: I know you are, but what am I?
Shelbyville Kid: A garbage man.
Bart: I know you are, but what am I?
Shelbyville Kid: A garbage man.
Bart: Takes one to know one!

*stunned silence*

Nerd: Checkmate!