My Youngest Son is a Chauvinist

Yesterday, my daughter came home from school with a home-made "Doctor Hat". When she told her brother what it was, he responded with "Don't you mean a Nurse hat?" I looked at him with surprise. Where did my son learn that from? We certainly didn't teach him that girls can only be nurses, did we? Rather than sit there and let him believe that, though, my wife and I explained that girls can be doctors and boys can be nurses too, if they want to be. He seemed to be satisfied with our explanation.