“He who can, does. He who cannot, teaches.” – George Bernard Shaw (Man and Superman, 1903)

What George Bernard Shaw didn’t say in today’s oft-repeated quotation — but should have — is that those who teach also do.

A friend of mine has been very successful as an executive in the financial-brokerage business. At a time when brokers are being laid off by the thousands, he has received a major promotion. He makes — I’m guessing — at least a quarter of a million dollars a year, and he’s on his way to doing better. I’d be surprised if he didn’t end up as CEO of one of the biggies.

I know him socially. He’s bright but not intellectual, funny but never a clown, knowledgeable but not showy, sociable but not your best friend. I have had discussions with him about the brokerage business, and it’s clear he knows what he’s doing — but his knowledge of buying and selling stocks is not his financially valuable skill.

So what is? What is the secret of his success? Why is he making big money? How can I be so sure he’ll rise to the top?

I had an inkling of an answer recently when, in a passing conversation about a speech he was preparing, I made a recommendation. He liked the idea and told me so — and in the weeks that followed, he sent me a series of e-mails asking for further details and clarifications on my suggestion.

I was happy to oblige. What is better fun than helping a worthy friend?

A month later, he called me and invited me over for a drink. He showed me a power-point presentation of his speech. It was very powerful. His use of the simple idea I’d recommended was very good — better than I could have done.

I realized that his financially valuable skill is his ability to teach. And his job, it turns out, is to travel around the country and teach brokers how to do things.

He isn’t an expert at what he teaches. He is an expert teacher.

If you can teach, you can make a lot of money. The secret is to teach not what you are interested in but what others are interested in learning.

This was the skill of the Greek Sophists. They traveled about teaching whatever subject the market demanded. In doing so, they engendered the scorn of idealists like Plato but the grateful appreciation of the knowledge seekers who paid them.

It’s also what motivational speakers do. Many of the best (Tony Robbins, for one) aren’t experts at motivation (the way, for example, Saul Gellerman is) — but they are brilliant at taking existing ideas and making them exciting and comprehensible. In other words, they are good teachers.  Do you know how much Tony Robbins makes? (Mucho.)

And motivation is just one subject in demand today. There are dozens of others — including personal power and productivity, art and gardening, and even public speaking. Pick up a daily newspaper and browse through it. You’ll get some ideas. If you can teach — and you aren’t satisfied with a teacher’s salary — perhaps you should consider other subjects.