Dai Song was a famous painter of the Tang Dynasty, and his paintings of oxen were highly cherished by scholars and connoisseurs.
During the Song Dynasty, a man surnamed Du from Sichuan province owned a painting of an ox by Dai Song. He treasured it greatly, often taking it out to admire it and even airing it in the sun on fine days.
One day, a young shepherd boy saw the painting and couldn't help but laugh. He pointed out that while the painting was meant to depict fighting oxen, the ox's tail was raised high in the air. From his own observations, he knew that during a fight, an ox tucks its tail between its legs. Therefore, the painting contained an error. Mr. Du agreed with the shepherd boy's assessment and subsequently valued the painting much less.