Ah yes, the old and sneaky vs the young and stupid.

My 16 year old and I play this game all the time. My two favorite responses:
". . . (long pause) . . . Ah crap, Dad. You did it again, didn't you?"
and
"I know, I know. You're trying to teach me to think before I speak. Darn it."

My all time favorite with ANY teen:
We were on a Scout camping trip, and the 15 year old was declaiming on the evils of nuclear weapons and and the wonders of unilateral disarmament. I was 'uhm-ing' in the right places. When he finally paused, I said "Yeah. It really is terrible, when all those people were killed with nuclear weapons, they were so much deader than they would have been with just a bullet!"

He responded with, "Yeah, that's right . . . wait, what?". He got the "what?" out just before the Assistant Scoutmaster and his father (who were both in earshot) started howling.

Ben