The 8 Monkeys
(This is reportedly based on an actual experiment conducted in the U.K.)
Put eight monkeys in a room. In the middle of the room is a ladder, leading to a bunch of bananas hanging from a hook on the ceiling.
Each time a monkey tries to climb the ladder, all the monkeys are sprayed with ice water, which makes them miserable. Soon enough, whenever a monkey attempts to climb the ladder, all of the other monkeys, not wanting to be sprayed, set upon him and beat him up. Soon, none of the eight monkeys ever attempts to climb the ladder.