Henry Kissinger, in his book The Whitehouse Years, tells of a Harvard professor who had given an assignment and now was collecting the papers. He handed them back the next day and at the bottom of one was written, "Is this the best you can do?"
The student thought, "no," and redid the paper. It was handed in again, and received the same comment. This went on ten times, till finally the student said, "Yes, this is the best I can do."
The professor replied, "Fine, now I'll read it."