A man goes to the zoo

He's at the gorilla exhibit when a gust of wind blows some dust into his eye. He rubs his eye to get the dust out and the gorilla sees this and immediately bends open the bars to the exhibit and beats the man unconscious.
The man wakes up in the zookeeper's office and the zookeeper asks him what he did to enrage the gorilla. "Nothing," he says, "I just rubbed my eye to get dust out of it."
"Oh, in gorilla language that's the equivalent of giving someone the middle finger," explained the zookeeper.
The man wasn't very happy with this explanation and began to plot his revenge. Knowing that gorillas will mimic human behavior, the man went to a store and bought one hot dog, 2 knives, two party hats, and two party horns.
The next day he returned to the zoo and set the items in a bag within arm's distance of the gorilla exhibit and waited until he got the gorilla's attention. Once the gorilla saw him he reached into the bag for a hat and placed it on his head. The gorilla did the same. Then the man grabbed one of the horns and started playing it. The gorilla grabbed the other horn and started playing the same tune. Finally, the man reached into the bag and grabbed the knife. Before coming to the zoo he had stuffed the hot dog down his pants and at this point he unzipped his fly and pulled the hot dog halfway out. He then cut it in half with the knife.
The gorilla reached into the bag and grabbed the knife. It looked down at its crotch, then at the man, then back at its crotch and finally back at the man before rubbing its eye.