Max learning to decipher gender

This morning Max was looking at our dog Jack and asked, "How do you tell if it's a girl dog?" I told him it's the same way you tell with people. He then asked if the way you could tell a boy dog was the same as you could tell with humans. I said, "what does a boy have that a girl doesn't?" After a long pause, he replied, "A shotgun." I don't know what you call it in your house, but he's referring to his plastic, toy shotgun, not anything else. So, then i said, what do boys have on their bodies that girls don't? and he said, "Boy clothes." We just left it at that.