Writing about something, even something you know well, usually shows you that you didn’t know it as well as you thought. Putting ideas into words is a severe test. The first words you choose are usually wrong; you have to rewrite sentences over and over to get them exactly right. And your ideas won’t just be imprecise, but incomplete too. Half the ideas that end up in an essay will be ones you thought of while you were writing it.
This post seems like a nice illustration of Paul Graham’s latest essay about how you don’t understand something until you’ve written about it.