can do or can't do?

     So tired of hearing the old maxim "Those who can, do, and those who can't, teach" used to refer to teachers--in the sense that anyone who is teaching probably doesn't know how to do anything--that I feel a need to explain what the quotation really means. It started with the military ( the army ). People in their 40's were drafted back into the army in WWII--people who had served in WWI, who had military experience. They were too old to be drafted for any purpose except to teach other people how to be soldiers. All the people young and fit enough were to be soldiers, and those too old to serve in this way were to teach them how to do it.  Those who could, did, and those who couldn't, taught. We won that war, by the way.