Two capuchin monkeys are in a cage, separated by a wall of wire. They can see one another clearly. Next, they’re given the same task—when the researcher asks, they are to hand over a rock. The first capuchin completes the task and in return is given a delicious grape. Now it’s the second monkey’s turn ... and the action begins.
He performs the task in identical fashion, but instead of being rewarded with a grape, he is given a bland slice of cucumber. This capuchin puts the cucumber in its mouth for a few seconds, then spits it out and throws the cucumber into the face of the researcher. The whole process is then repeated.
This time the capuchin who received the cucumber watches the grape-receiving capuchin closely. And on their turn, they take care to do the task perfectly, mimicking their counterpart in every way, even tapping the rock against the side of the cage to make sure nothing is wrong with it.
After handing it over, the capuchin is given a cucumber just the same. This time he really loses it.
The cucumber is thrown into the face of the researcher immediately. The capuchin then climbs the wire wall between the two cages, shaking it aggressively, desperately trying to draw attention to the fact that for the same task, the pay was unequal.