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 because they're too lazy to cut them. Girls are attracted to boys, even at an early age. At an early age, boys are attracted to dirt.
Most baby girls talk before boys do.
Before boys talk, they learn how to make machine-gun noises. Girls turn into women. Boys turn into bigger boys.