What is "sarcastic"?

James often likes to use sarcasm with the boys.  Use to drive Nathan crazy but he's slowly gotten used to it.  Now Evan is curious what "sarcastic" means and we've explained to him it's when you say the opposite of what you mean.  He managed to use it in a sentence quickly after that.

"I don't like Logan, and I'm not being sarcastic".

I can't remember exactly what Logan did to upset Evan, but it was quite amusing despite the dispute.