God, thank you. I really dislike the old aphorism that if you can't teach something you don't understand it.

Teaching is a whole complicated skill unto itself, especially if one is teaching to beginners. Like (since we're on HN), how easy is it to imagine someone very good at programming but would be a terrible choice as a Comp Sci 101 prof? I'm guessing "very."

The whole idea deeply undermines teachers of all subjects.