Why do gorillas have nipples??

How in the world does my 6 year old's brain work??? First he asked if a gorilla was half man. I tried my best to explain that no, a gorilla is a gorilla. Then he asked "Why do gorillas have nipples?" Then he wanted to know why boy gorillas had nipples. The fact that they just do, isn't good enough. He wants specifics. I think it's time to check out a gorilla book.