Sarcasm and Our Six Year Old

I better write this funny down before we forget.
Isaac being just six years old is a comedian in our book. He doesn't try to be funny, but he just is.
He and I were busily grabbing a few Easter items at Walgreen's early last week. The clerk at the register noticed he looked rather bored, and started having him bag the candy to keep him occupied. Isaac feeling playful decided to shut the bag so the clerk could not get any more items in it. Mr. Walgreen's could tell he was playing and said "Nice job keeping that bag open." Before I could even say -- "hey, bud please open the bag"- Isaac had moaned with disgust and simply stated- "SARCASM!" and sat down by the bags no longer willing to help or hinder the bagging.
Neither myself or the clerk could hide our surprise and smiles at his clever outburst. How in the world did he know what that meant, and well enough to use it in context? We asked him that very question, and he said "AJ taught me."