I always heard (from a teacher, in a self deprecating fashion): Those who can, do. Those who can't, teach. Those who can't teach, teach others to teach. Tho

I always heard (from a teacher, in a self deprecating fashion): Those who can, do. Those who can't, teach. Those who can't teach, teach others to teach. Those who can't teach others to teach become school/Uni administrators. ... In reality I think it's hard to teach stuff you can't do, and teaching it becomes a great way to learn how it actually works.