The Clever Judge's Donkey is a proverbial tale or, if you prefer, a little folk story.
It speaks of an ancient time when a merchant who led his donkey into an inn lost his money, and the chances were high that the thief was still inside the inn. The local magistrate gathered everyone in the inn and told them that the merchant's donkey was a divine creature that could find the thief.
Then, the magistrate took the donkey into a small, dark room and had people touch the donkey's tail. If they grabbed the tail and the donkey stayed silent, they were innocent; if the donkey brayed while being touched, they were the thief.
Of course, the divine donkey was a farce. The truth was that the magistrate had smeared ink on the donkey's rear end, and in the end, he had everyone who entered the room show their hands—the ones with ink were innocent, while those with clean hands were the thieves.