Baseball Bat or Animal Bat

Okay, you human, let's break this down. You thought hitting a ball was normal. You probably thought a bat was used to hit a ball. This is not how it works, you dummy. This is a play on words. The joke is funny because it implies that you hit a "bat", the animal, and not "a bat", the baseball equipment. The idea that someone is abusing an animal, like a bat, is not funny at all, but the way the sentence is written is funny.