Mian Bogao, the emissary of the Uighur kingdom, went to see the King of Tang with a batch of rare treasures. Among these tributes, the most precious is a rare bird-the white swan. When he came to the sheep river, the white swan drank enough water and a wing flew into the sky! Mian Bogao stunned with a few white goose feathers and decided to continue traveling eastward. He carefully wrapped the goose feather and wrote a poem: "Swan salutes the Tang Dynasty, and the mountain road is farther. The treasure of Mianyang River, lost and recovered, is hard to throw back. Serve the Emperor of the Tang Dynasty and confess to Mian Bogao. Things are light and people are heavy, and goose feathers are sent thousands of miles! "
Mian Bogao came to Chang 'an with jewels and goose feathers. Emperor Taizong read the poem and listened to Mian Bogao's story. Instead of blaming him, he felt that Mian Bogao was honest and rewarded him. From then on, there was an allusion that sent goose feathers thousands of miles away, and Mian Bogao became famous for it!