those who can't do, teach

those who can't do, teach

Those who are unable to successfully find a career in their field of interest end up teaching about it instead. (A shortening of "Those who can, do; those who can't, teach.") A: "I know he always aspired to be a great novelist, but the last I heard, he's still teaching middle school English." B: "Well, those who can't do, teach."
See also: teach, those, who