I'm not sure where this sassy pants of a son came from, because his big sister isn't sassy at all. So he hasn't had this sort of behavior modeled for him often. Still, he's full out attitude about 60% of the time these days. Typical example:
J is busy wildly hitting the bottom of JT's bouncy seat (and the toy bar above it) with his dinosaur, so I calmly remind him that we don't hit things that the baby is in. He ignores me, so I tell him again.
Me: "J! Mom just told you not to hit JT's chair while he's in it. You need to stop now."
J (in his best sassafras tone complete with eye rolls and head shaking): "Mom. I am not going to hit baby JT with my T-Rex. Calm down."
Me: "Ummm...yes. I will "calm down" while you take your sassy mouth to time out."