雁足留书 — “A letter attached to a wild goose's foot”

During the Han Dynasty, Su Wu was sent as an envoy to the Xiongnu territory, where he was detained. Later, when Emperor Zhao of Han sought a peaceful alliance with the Xiongnu and requested the return of Su Wu and other Han envoys, the Xiongnu falsely claimed that Su Wu had died.

However, another Han envoy visited the Xiongnu. A Han official named Chang Hui, who had been captured earlier, managed to bribe a guard and secretly met with the Han envoy at night.

When the Han envoy met with the Chanyu (Xiongnu leader), he fabricated a story, saying that the Emperor had been hunting in the Shanglin Garden and found a letter from Su Wu tied to the foot of a wild goose. This letter supposedly stated that Su Wu was still alive and held captive.

Faced with this clever deception, the Chanyu had no choice but to admit that Su Wu and the other Han captives were indeed still alive, and eventually allowed them to return to the Han Dynasty.