There was a family which had twins, one pessimist and one optimist. The pessimist always complained of something: either the food is too hot, or that it's not sunny outside; on the contrary, the optimist looked forward to everything, even some little things made him happy and he found easily a reason to be in good mood.
For Children's day the parents were thinking to give them some presents and to test their behavior. So in the room of the optimist they put a dung pie and in the room of the pessimist they left many beautiful toys.
The pessimist entered his room and look at each toy, then began to cry "this toy should be green and the train had to have three cars and no five!"