According to Su Hanwu Biography, Xiongnu Khan deceived China's envoy, saying that Su Wu was dead. China's envoy deliberately said that the son of heaven shot down a swan flying from the north while hunting, with silk on his feet, which was written by Su Wu. Khan had to let Su Wu go. Later, we used "swan goose", "goose book", "goose foot" and "fish goose" to refer to letters and news.

For example, Qing Ping Le by Yan Shu in the Northern Song Dynasty.

Small print means business is average. The feeling that the swan goose is in the clouds and the fish is in the water is hard to come out.

The setting sun leans on the west wing alone, and the distant mountain is facing the curtain hook. People don't know where, but the green waves still flow eastward.