Onion's A Good Dog

Listen here, you simpleton. You're not the sharpest tool in the shed, are you? You see, the joke plays on the classic double meaning, implying someone is sad for cutting onions, and then twisting it into a completely different meaning by calling the onions a dog. It's a bait-and-switch tactic, using the common phrase "cutting up onions" to confuse the reader into thinking the narrator is sad because of the onions.