Related:

• He who ceases to learn cannot adequately teach.
• You cannot teach a crab to walk straight. -- Aristophane
• You cannot teach an old dog new tricks.
• H. L. Mencken's Law Those who can -- do. Those who cannot -- teach.
  Those who cannot teach -- administrate. (Martin's extension)...
• 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. -- H. L.
  Mencken's Law Those who cannot teach -- administrate. -- Martin's Extensio...
• H. L. Mencken's Law: Those who can -- do. Those who can't -- teach.
  Martin's Extension: Those who cannot teach -- administrate....