The saying that geese can deliver letters comes from Su Wu. "Han Shu" records: "In a few months, Emperor Zhao ascended the throne. For several years, Huns have taken a liking to Han people. Han asked for martial arts, etc., and Xiongnu bullied him. The envoys of the later Han Dynasty returned to Xiongnu, and Chang Hui invited them to keep pace with the defenders, so as to meet the envoys of the Han Dynasty at night and come from Chen Dao. The messenger called Khan and said that the son of heaven shot in the forest and got geese, full of silk and silk, and his words were strong in a river. " When Su Wu was sent to Xiongnu, he was detained by Khan. Ten years later, when the Han Dynasty approached the Xiongnu, Chang Hui designed the Chinese ambassador to tell Khan that the emperor of the Han Dynasty had shot a wild goose while hunting, and a letter was tied to the goose's foot, saying that Su Wu was herding sheep here in Xiongnu. This just let khan had to put Su Wu back.

The saying that fish can spread books comes from Yuefu poem "Drinking Horses in the Great Wall Cave": "Guests come from afar and leave me a pair of carp. Hu Er cooks carp with books in it. " It means that when you cut the fish, you saw a letter in the belly of the fish, so the letter was later called "fish book". This allusion is also widely used by everyone, such as "the dream of closing the mountain is long, the voice of the fish geese is few" and "What is the purpose of sending the fish book?" The water is far away and the mountains are everywhere. "

Fish-goose biography is synonymous with letters. Because of this historical heritage, people will print the shapes of fish and geese on stamps.