A young shepherd boy was watching over a flock of sheep near his village. He got a bit bored, so to liven things up he shouted “Wolf! Wolf!” just to have some fun.
The villagers ran to help him, but when they arrived they couldn't see a wolf. The boy laughed at them and they left in anger.
A couple of days passed and the boy decided to have some fun again and so he thought he'd try again. He shouted “Wolf! Help! Please!"
This time fewer people from the village came to see if there was a wolf. And once again they found no wolf and left angry at the shepherd boy.
The next day, as the boy was watching the sheep, a real wolf came. Now the boy started screaming and shouting, terrified of the wolf: “Help, help! Please, help me! There's a wolf and he's killing your sheep!”
But no one came, because the villagers didn't believe him. This is how liars are rewarded - even if they tell the truth, no one believes them.