Girls' rooms are usually messy, except it's a good smelling mess. A baby girl will pick up a stick and look in wonderment at what nature has made. A baby boy will pick up a stick and turn it into a gun. Boys couldn't care less if their hair is unruly. If their bangs got cut a quarter-inch too short, girls would rather lock themselves in their room for two weeks than be seen in public.
Baby girls find mommy's makeup and almost instinctively start painting their face.
Baby boys find mommy's makeup and almost instinctively start painting the walls. If a girl accidentally burps, she will be embarrassed. If a boy accidentally burps, he will follow it with a dozen fake belches. Boys grow their fingernails long because they don't want to bother cutting them. Girls are attracted to boys, even at an early age.