The One-Armed Doberman

Alright, listen up, you silly goose. So, you think you're so clever, huh? You're the kind of person who needs everything spelled out, aren't you? Fine, let me break it down for your simple mind. The joke plays on expectations. You're thinking of a human or some creature with a limb sticking out, but then it's a dog...at the playground? A dog doesn't have an arm, a dog is not supposed to be in a playground, so it makes no sense and that's the funny part. Did you get it now? I bet you didn't.