I remember when my older son was a baby and I couldn't wait to find out what he was thinking about. And then he started talking, and I knew what he was thinking during every waking moment. It was strange, this intimate knowledge of someone else's inner life, but I loved it. And then suddenly he started talking about things I knew nothing about, and I realized he had a life that didn't involve me in every moment anymore.
It happens with every child. Eventually they develop an interest in something you know nothing about, and you suddenly know that at some point that child will have a whole big wide life that isn't anchored to you.
Maybe it's dinosaurs. Or Greek myths. With my son it was baseball statistics. He was five years old, and he became obsessed with baseball and baseballs stats. I don't like baseball and I have no interest in baseball stats, so I wan't even tempted to try to cram to catch up to him. I just let him tell me what he knew, and feigned interest so he knew I always wanted to hear what he had to say. And you'll listen and marvel at where your child's interests lie, and let them talk and talk and talk (even when you really don't care about the content of what they're saying).
One of my friends attended her son's PhD dissertation defense last week. Someday this will be you. Or maybe it won't be a dissertation defense, but it'll be a promotion at work, or some sort of award, or even just listening to your kid explain something they're really good at to someone else. You'll be just as proud then as you are the first time, and a lot less surprised.
It's good when you get to the part of the conversation in which your child is teaching you new things.