There’s an old saying that goes like this:

Those who can, do.

Those who can’t, teach.

And those who can’t teach, teach gym.

Now, Gov. Chris Christie isn't going that far in disrespecting gym teachers, but he is saying that math and science instructors should be paid more than physical education teachers.

Why? Because schools – and students – need them desperately.

While Christie’s remarks might be an affront to the merit-pay-averse New Jersey Education Association, which likes it better when everyone is locked into a rigid pay scale, the governor isn’t a revolutionary. He didn’t invent teacher merit pay.

Or capitalism.

After all, isn’t that how the market determines salaries? If there were a shortage of engineers or a demand for more of them, for instance, the salaries of those professionals would rise.

So, to persuade more people to become math and science teachers, Christie is proposing paying them more – more than gym teachers (and maybe more than music and art and history and English teachers).

It makes sense.

But it led Steve Wollmer, spokesman for the NJEA, to ask, “What’s he got against gym teachers?”

Christie's response: "Cut the crap."

Wollmer’s return volley: “Juvenile.”

Wollmer says “each teacher contributes equally to a students’ success.” But that’s silly. Do bad and good teachers contribute equally? Of course, not. Not all teachers are equal. You could even say not all good teachers are equal.

Wollmer adds that physical education teachers must know physiology and other difficult subjects. But that’s not relevant. The issue is whether, based on demand, teachers should be paid equally, not who had the toughest course load before getting certified.

In a nation where obesity is an epidemic, Wollmer can argue that physical education instructors and math teachers are equal. It’s silly – and if he truly believes it, naïve.

But it also misses the point: There is no shortage of gym teachers.