To teach someone something, you want to meet them where they are on the road and then help them along for the next few steps.
That’s two hard things: understanding where they are and then teaching them the next thing. We understand that the teaching part is hard. That’s what we tend to work on. But the meeting-them-where-they-are part has its own set of problems.
When I try to write about something technical, I constantly find myself asking: What if the reader doesn’t don’t know about this yet? I can’t define thing C in terms of thing A and thing B if the reader doesn’t know about thing B yet.
“If you wish to make an apple pie from scratch, you must first invent the universe.” – Carl Sagan
Of course, you have to start somewhere. And your audience will quickly lose interest if you start several levels of abstraction beneath what they’re actually interested in.
But I find it helpful for my own understanding to keep asking myself this question. Questioning my understanding often shows me the gaps in it. Trying to write about very basic stuff nearly always turns out to be harder than I expect. Sometimes I learn things I’ve missed or forgotten. Sometimes I turn my intuitive knowledge into knowledge that’s more concrete and communicable.