• He who can, does. He who cannot, teaches.

    George Bernard Shaw: He who can, does. He who cannot, teaches.
    Man and Superman "Maxims for Revolutionists" (1903). A further extension appears in Jacob M. Braude, Speaker's Encyclopedia of Stories, Quotations, and Anecdotes (1955): "Those who can, do; those who can't teach; and those who can't do anything at all, teach the teachers."