“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.”