I have three children, a son (currently 6) and twin girls (3-and-a-half). More precisely, identical twin girls. Split from the same zygote, they have the same DNA. And yet, they are utterly different people. From the first hours of their lives, we began to notice differences. B wanted to cuddle up with her sister. E wanted to finally get some space to herself. One slept easily and beautifully from day one, the other needed a lot of cuddling and reassurance before she would sleep, and she always (always) wakes up first whether from nap or bedtime. The lifetime hours of sleep she has robbed from her sister would be an amazing number.
In this, their third year, we have noticed something remarkable. B is in many respects exactly like our son. More like him in fact than her genetic replica and constant companion since birth. She has his personality, the same hangups about how things are done, the same sleep struggles. Just like him, every button on a shirt must be fastened and every zipper pulled all the way to the top. They will both only pee in one bathroom, and only poop in the other. They play together beautifully, both wanting to run around and crash into things while our other daughter plays dolls or house or dress-up.
Children are such a wonder. They are born and you think you know everything about them. A few years later, and you just watch in awe at all the things contained in these mysterious little people.