In "Mathematical Apocrypha Redux", Steven Krantz tells the following tale:
A student of Plato (428 B.C. - 348 B.C.) once asked the great master, "What practical uses do these theorems serve? What is to be gained from them?" Plato's answer was immediate and peremptory. He turned to one of his slaves and said, "Give this young man an obol [a small Greek coin] that he may feel that he has gained something from my teachings. Then expel him."
The story is repeated at King's Way Classical Academy. What is the origin of this story? Is there compelling evidence that it actually happened?