[cc2k]

Army MP Retired wrote:

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There’s an adage I once heard: those that can’t do, teach; those that can’t teach, write text books.

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I always heard it like this:

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

And recently, someone here of FreeRepublic added:

Those who can't make it as school administrators run for congress.

[BenLurkin]

Those who can, do.

Those who can't, teach.

Those who can do neither, criticize.