Our 10 year old has discovered sarcasm, and he's very good at it

Mostly it's funny. 'Wow, dal again, what a surprise.' 'Great, another family function.' My partner and I laugh, which I know is part of the problem. Yesterday he tried it on his grandmother, who had asked a perfectly ordinary question about school, and she didn't laugh. She looked hurt, and he looked confused, as if he couldn't see why it landed differently. I don't want to stamp out the humour. It's one of the things I like most about him right now. But he can't see the line between joking with us and being rude to someone who isn't in on the joke. How did you explain that to yours, short of a speech about respect?