Monkeys are unfairly rewarded --- their reaction amazes observers!

The experiment in the video will make you smile but also invites a lot of reflection. 

Two monkeys are placed in two adjacent cages; each of them must perform a simple exercise to get a reward. The exercise consists of picking up a stone and giving it to the female researcher, thereby receiving as a reward a small piece of cucumber. 

As long as BOTH receive the same vegetable, a piece of cucumber as the reward , everything works smoothly; but look what happens when the monkey on the right starts to get grapes instead of cucumber! Suddenly, the monkey on the left stops and starts to protest because it feels that what is happening is unfair......