Me (to elder son Thomas, aged 5): <jokingly> You silly moron!
Thomas: YOU'RE a moron!
Me: You don't even know what a moron IS!
Thomas: Yes I do, Daddy. It's a kind of bird!
Me: <after 5-second pause with very puzzled expression on face> MorON! Not MoorHEN!
He was nearly right, I suppose. I AM as bald as a coot...