When a baby is born, the obstetrician or midwife announces “It’s a boy” or “It’s a girl.” As toddlers, children learn to classify everyone as either boy or girl. When our firstborn was very young, we overheard her talking to herself as she grappled with the concept:
Let’s see… I’m a girl, and Kimberly [her baby sister] is a girl, and Mommy’s a girl… but Daddy’s not a girl… He’s a boy. [Pause followed by exasperated sigh] Cause he doesn’t know any better!
As with most things in science, the concept of boy versus girl is more complicated than it appears at first glance. It’s not a simple dichotomy. We humans like to classify everything into neat pigeonholes, but Nature’s inventiveness outsmarts us at every step.