My nephew (10 now) when he was 8, I asked him to define sarcasm,
   because he was starting to use it on me (with appropriate tone
   of voice). He said, "Saying the opposite of what you really
   mean". It was a good definition. I asked him to not use it with
   me. He does now again, but he lost the tone-of-voice bit and now
   it's more of a "you're in on the joke too" dry-wit than straight
   sarcasm.