Definition of a girl

On the way to school every morning, we pass a little field with a horse. This morning when we passed the horse, Jacob said, "Mom, when's that girl horse going to have a baby?" I asked, "how do you know it's a girl." He said, "When we went for a walk and stopped by the horse, I saw that it had long eyelashes and a long tail...so when's she going to have a baby?" So by Jacob's definition of a girl...am I a girl? Hopefully this only applies to horses.