Is the comeback I know you are but what am I good?

Yes, because in most cases people aren't witty enough to have comebacks to that. B

e prepared for combacks to that though. Such as:

You could say:

Person 1: You're an idiot.

Person 2: I know you are but what am I?

Person 1: I already told you, you're an idiot.

or

If I am, what are you?! Oh WAIT, I know! A ___ (fill in blank).

or

That's the best you got? A little kindergarten insult? How sad....

or

I don't speak ___(Word of your choice, maybe baby, or stupid, etc).

Say this: Say, how old are you? 5!?

Say: Are you serious right now? Then laugh and walk away, making them feel like an idiot.

Thanks, that's a compliment in comparison to what you are.

You could say to that: I know you are, but what am I? Oh WAIT! I know! SOmething YOUR not. Hope this helps! :)