liecanthrope

"It's a children's story, about a young shepherd boy who gets lonely while tending his flock. So he cries out to the villagers that a wolf is attacking the sheep. The people come running, but of course there's no wolf. He claims that it's run away and the villagers praise him for his vigilance."

"Clever lad. Charming story."

"I'm not finished. The next day, the boy does it again, and the next too. And on the fourth day a wolf really comes. The boy cries out at the top of his lungs, but the villagers ignore him, and the boy, and his flock, are gobbled up."

"Well, that's a little graphic for children, wouldn't you say?"

"But the point is, if you lie all the time, nobody's going to believe you, even when you're telling the truth."

"Are you sure that's the point, Doctor?"

"Of course. What else could it be?"

"That you should never tell the same lie twice."