Sometimes people ask you a question that you don't want to answer, or make a statement to which you'd rather not respond in any tangible way. Sometimes people even have a self-righteous fit, flinging accusations and unjustified interrogation hither and yon. As a kid on a playground, it's easy to come back to stuff like that: "I know you are, but what am I?" You have to put that snerky sing-song inflection into it for it to work. It usually accomplished its job of making the other kid want to punch you, which often shut them up, and at least distracted them from the merits of whatever eight-year-old point they were trying to make.
We're all grown up now (I think), and that won't work anymore. Or will it? If you tweak the language a bit and update the attitude, you get my new all-purpose comeback/dodge/piss-off-the-self-righteous. Here's how it works:
You put on a concerned, thoughtful face. Then, in a mildly condescending, knowing tone, you say "I think that says more about you than it does about me." And since we're adults now, you can improvise a bit and adapt it to the specific situation for maximum effect.
This will work for almost any scenario.
Co-Worker: "Where's my lunch? Who took my damn lunch? You! Did you take my lunch?"
You: "You think I took your lunch?" Shakes head slightly. "I think that says more about you than it does about me."
Pencil-Pushing Bureaucrat at Work: "You didn't fill out the forms you were supposed to. We can't process the reports without that form."
You, smirking: "I think that says more about you than it does about me."
Beefy Guy At Bar: "What the eff were you looking at? Were you looking at my ass? I'm gonna break you in half."
You, drawing your concealed weapon: "I think that says more about you than it does about me, beautiful." Blow kiss.
Right between the goalposts!
It's guaranteed effective, although it helps if you don't mind wearing body armor under your everyday clothes. And some kind of facial protection.