Those who can, do; those who can't, teach.
Prov. People who are able to do something well can do that thing for a living, while people who are not able to do anything that well make a living by teaching. (Used to disparage teachers. From George Bernard Shaw's Man and Superman.) Bob: I'm so discouraged. My writing teacher told me my novel is hopeless. Jane: Don't listen to her, Bob. Remember: those who can, do; those who can't, teach.