Those who can't do, teach.

A sly jab at the gap between talking about a skill and actually having it: that is the sting in Shaw's line. It comes from his play Man and Superman, where he wrote something closer to "He who can, does; he who cannot, teaches," and the popular version is a looser paraphrase. Shaw was a satirist who distrusted pretension, and he enjoyed poking at anyone who gained authority from explaining rather than doing. Yet he was also a lifelong lecturer and public persuader, so the remark is better read as mischief than as a considered verdict. Plenty of excellent practitioners teach well, and many fine teachers have never needed to prove themselves elsewhere.

Still, there is a useful point underneath. Knowledge that has never been tested can grow comfortable and vague, and we are right to be a little wary of advice from people who have never tried what they recommend.

Someone today could use the line as a private check. If you want to run a workshop on public speaking, give a talk first, even a clumsy one. If you already teach, keep a small practice of your own going, so your lessons come from experience rather than memory.