“Those who can -- do. Those who can't -- teach.”
“Those who can do, those who can't teach.”
“Those who can't do, teach. And those who can't teach, teach gym.”
“To my mind, those who can, do. Those who can't, gripe.”
“Those who can, do; those who can't learn classification and cataloguing.”
“Those who know, do. Those that understand, teach.”