It's Ad Hominem!

Drew and Anna were having a discussion (also known as a sibling argument).

Drew said, "I'm older than Zipper and I'm more important."
Anna said, "Zipper is older than you."
Drew said, "That's stupid."
Mom interjected (with only the tiniest bit of sarcasm), "Drew, that's a really brilliant way to make your point and continue the discussion."
Drew replied, "I know, it's ad hominem."

I love our school district!