THE SHEPHERD'S BOY AND THE WOLF
A shepherd's boy was tending his flock near a village. He wanted to have fun; he made a plan; he would play a trick on the villagers by pretending that a wolf was attacking the sheep. So, he shouted out, "Wolf! Wolf!". When people heard him, they came running, and the boy laughed at them. Then, on another day, he played the same trick, and the people came running again but there was no wolf. At last, a wolf really did come and the boy cried, "Wolf! wolf!", but the people were used to hearing him shout "wolf", so they did not think he was serious. And things were very easy for the wolf; he killed all the sheep one by one without a hurry.
You cannot believe a liar even when he tells the truth.