As a parent of children who are now young adults, I'm always surprised and amazed at how much they know. They speak confidently and at length on topics about which I know nothing. One discusses "optimality theory in relation to phonology and historico-linguistics" with the same enthusiasm in her voice that she once had for the latest version of "My Little Pony". The other shares jokes with her friends about "spin ice", which has nothing to do with amphetamines and a lot to do with magnetic particles.
It has taken them many years of study in their chosen fields to be able to do this. Nevertheless, it doesn't seem so very long ago that they were coming to me with their questions. Until the middle of primary school at least, my knowledge of the world generally exceeded theirs, and I was able to answer them. Or, like Calvin's Dad in "Calvin and Hobbes", I could bluff my way out of a sticky question.
My ignorance of their areas of expertise is not a source of sadness. I love listening to them. I learn lots of interesting things from them. It's thrilling that they've benefited from all those years of study and are still interested and excited about what they're learning.
I'm also aware that in some areas, such as human biology, my knowledge still exceeds theirs. It seems to me that most of us have knowledge which is not common knowledge. Plumbers talk to other plumbers (and sometimes their customers) about equipment and techniques that few others are familiar with. The chef at the local restaurant knows more about food and cooking and catering than I'll ever know. The elderly couple who live a few doors down my street know a vast amount Poland because that's where they were born and raised, but no-one else in the street has that knowledge.
It's amusing to think that as children we assumed that one day we'd know everything there is to know, just like our parents. (Well, didn't you?)