A city in the Netherlands had a problem with litter. The sanitation department tried doubling the littering fine and even increasing the number of litter agents who patrolled the area, but to no avail. Then someone suggested that instead of punishing those who littered, they could reward people who put garbage in trash cans. A plan to devise a trash can that could dispense coins when litter was inserted was rejected as too expensive. But it led to another idea: the sanitation department developed a trash can that played a recording of a joke when refuse was deposited! Different cans played different kinds of jokes, and the recordings were changed every two weeks.
Citizens went out of their way to put garbage in trash cans, and the streets were clean again.