&%$@ words

Cal has been saying "dangit" when he's frustrated. I told him that he should say, "Oh, man!" because it sounds a lot better coming out of a kid's mouth. Then today:
Cal: We also can't say "stupid?"
Me: Who told you that?
Cal: Ella.
Me: Well, you can say "stupid." You may not call anyone stupid, but if something really is stupid, then you can say it's stupid. [pause] Although, if I say, "It's time for bed, Cal," it wouldn't be kind if you said, "That's stupid."
Cal: If my name really was stupid, then you can say that I'm stupid! [smiling]
Me: True.