An old student's joke - A student always fails exam. And takes extra classes - the professors explains and explains. But the student - "I don't get it, I don't

An old student's joke - A student always fails exam. And takes extra classes - the professors explains and explains. But the student - "I don't get it, I don't get it". Finally the professor loses his mind "Oh, come on - I already got it from that much explaining"