A dentist spotted a deadbeat patient while dining at his country club one evening. He called the patient aside, reminded him that he owed him $250 for work done more than two years earlier, and insisted the man pay up. To the dentist’s astonishment, the patient pulled a checkbook from his pocket and wrote a check to the dentist for the full amount.

Skeptical about the patient’s good faith, the dentist went directly to the bank the next morning and presented the check for payment. The teller handed it back with the explanation that the patient’s account was a little short of the amount of the check. Following a few minutes of good-natured conversation the dentist learned that the man’s account was twenty-five dollars short of the needed amount. The dentist smiled, went to the customers’ desk for a few minutes, came back to the teller, deposited thirty dollars to the account of the patient, and then again presented the check for $250 and walked out with a net gain of $220.

— Ralph L. Woods, How to Torture Your Mind, 1969