Those who can, do.
Those who can't, teach.
Those who repeat that stale insult to teachers risk learning a lesson in manners.
But fear of post-classroom discipline for sassing a teacher isn't the only reason not to reprise that can-do, can't-teach canard, an H.L. Mencken variation on a George Bernard Shaw theme.
You should be grateful to those who can - and do - teach. And if you think their job is easy, you should try doing it and learn otherwise.
Yes, teachers, like elected officials, business executives, college baseball coaches, real estate agents, truck drivers, accountants, plumbers, professional wrestlers and all other working stiffs, run the gamuts from terrific to awful, from motivated to unmotivated.
That doesn't mean, however, that all past, present and future problems in U.S. public education are teachers' fault.
Over the last decade or so, teachers have been increasingly overloaded with red tape and testing - and with unrealistic demands from us non-teachers.
Along the stressful way, teachers must handle both the parents who are too pushy and the parents who educationally neglect their children. All the while, teachers must try to get through to often-unmotivated and disruptive students.
In my distant youth, moms and dads generally - and reflexively - backed teachers in disputes against their own kids.
These days, too many moms and dads are too quick to buy their progeny's pitches that they are the victims of teacher-inflicted injustice.
Also these days, many teachers are getting out of that relatively low-paying occupation.
As reported in Saturday's Post and Courier, S.C. public schools are facing a teacher shortage. Though some of that personnel gap stems from a large percentage of teachers advancing into retirement age, we're also losing good teachers to bad vibes.
That's why it was encouraging to read this first sentence in a story from learned Post and Courier colleague Amanda Kerr in Tuesday's paper:
"More teachers feel the climate of their school has been positive during the 2013-14 school year compared with the previous year, according to a new survey released by the Charleston Teacher Alliance."
Still, many teachers here and elsewhere are understandably wary of having their salaries linked to student performance. In fairness to the teachers who draw weak academic hands in the kids assigned to them, such performance-based compensation should be based on a realistic curve.
And in fairness to us all, school administrators and pandering politicians should stop trying to re-invent the educational wheel.
It sounds reassuring that many school districts, including Charleston County's, have re-emphasized literacy in recent years.
Then again, why wasn't literacy emphasized all along?
Why were expert studies needed to re-confirm its building-block importance?
As for the uncommon bitterness in the ongoing debate about national Common Core standards, both sides could benefit from more common sense before making wild accusations against each other.
Now for this non-teacher's, non-expert, back-to-basics education-reform plan:
Flunk the flunkers: Sure, it's tough on the kids who get held back. Yet it's even tougher on them - and their classmates - to put them in over their scholastic heads.
Take charge: If defiant students force a clash of wills, teachers - and administrators - must assert themselves as the intimidators, not the intimidated.
Learn from teachers: The folks who make school policies from on high should heed the advice of the folks down in the classroom front.
OK, so those are simplistic solutions to complex problems.
But some educrats - and there's no apparent shortage of that group - make the mission of teaching more convoluted than it needs to be.
At the risk of appearing biased, long experience as a close relative to three generations of teachers has taught me that most of the folks in their occupation work hard.
They even bring their work home with them.
They also bring heartaches home about youngsters suffering rough times - youngsters lucky enough to have teachers who care about them.
And many teachers - and yes, school administrators - will share parents' pride and joy during the next 10 days of local high school graduation ceremonies.
That reward isn't confined to knowing that some of their scholars are moving onward and upward to elite institutions of higher learning like Clemson, Stanford and Harvard.
It also stems from educators' realizations that they have helped students of wide-ranging abilities take positive steps toward their academic - and personal - potentials.
So congratulations to the graduates - and the teachers.
And if you think teachers are those who can't, think again.
Frank Wooten is assistant editor of The Post and Courier. His email is email@example.com.