the shepherd boy and the wolf (cry wolf - no-one believes a liar even when he tells the truth)

A shepherd boy tending his flock one night amused himself by shouting to the local villagers, "Wolf, wolf!" when there was none, just to see them come running. He repeated the trick and each time the villagers came to help, just to find the boy had tricked them again. Later that night a wolf really did come. The boy again called "Wolf, wolf!", but believing it to be another trick, no-one came, leaving the wolf free to kill all the sheep he wanted.