Those who can, do. Those who can't, teach.

Those who can, do. Those who can't, teach.

Those who are especially skilled in a certain field or area will be able to pursue a career, while those who are less skilled will end up teaching about it instead. 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, do; those who can't, teach."
See also: those, who