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• Those who can, do. Those who can't, teach. Those who can't teach, teach teachers.
• Those who can, do. Those who can't, teach. Those who can't teach, criticize. -- Solomon Sho
• Those who can, do. Those who can't, teach. Those who can't teach, hack.
• Those who can, do. Those who can't, teach. Those who can't teach, write textbooks.
• Those who can, do; those who can't, teach. Those who can't teach, administrate.
• Those who can, do. Those who can't, teach. Those who can't teach, teach gym.
  -- Woody Allen, "Annie Hall...
• H.L. Mencken's Law: Those who can -- do. Those who can't -- teach.
  Martin's Extension: Those who cannot teach -- administrate....
• Those who can, do. Those who can't, teach. Those who can't teach, administrate.
  Those who can't administrate, consult. Those who can't consult, run for political office....
• Those who can, do. Those who can't, teach. -- H. L.
  Mencken's Law Those who cannot teach -- administrate. -- Martin's Extensio...