In ancient times, letters were written in silk and folded into the shape of carp. In the Tang Dynasty, Li Shangyin wrote a poem "Message to Duke Hu Ling": "Song and Shu were separated from their hometown for a long time, and the two carp were separated by a piece of paper. Mo Wen Liangyuan is an old guest, and Maoling is sick like autumn rain. " The ancient Yuefu poem "Drinking Horses in the Great Wall Cave" has the words "Guests come from afar and send me double carp".
In the Han Dynasty, Su Wu sent envoys to Xiongnu and was exiled to Beihai to shepherd sheep, so his contact with the imperial court was interrupted. Su Wu wrote a letter and tied it to the goose's leg, taking advantage of the habit of waiting for birds in spring in the north and autumn in the south. The emperor didn't know about Su Wu until the goose flew to the royal garden of the Han Dynasty. Therefore, the court took him back through diplomatic channels.