Li Shimin asked when the Tang Dynasty died, and Yuan Tiangang asked when pigs could climb trees.

Emperor Taizong recalled from Zhangzhou Du Yan, Wang Jue and Wei Ting who were exiled because of being implicated by Prince Li, and learned from them that Yuan Tiangang was a divination prodigy in Shu, so he sent a message to Yuan Tiangang asking him to meet in Chang 'an. Emperor Taizong summoned Yuan Tiangang in the Xiande Hall of Jiucheng Palace and praised his art.

Once Emperor Taizong asked when the Tang Dynasty would perish. Yuan Tiangang refused to reveal the secret for fear that the emperor would be angry. In the eighth year of Zhenguan, Emperor Taizong once again asked when the Tang Dynasty would perish. Yuan Tiangang said: When pigs can climb trees. Say that finish, Yuan Tiangang retired, and Yuan Tiangang died in the same year after he retired.

Later generations' answer to the question that pigs in Yuan Tiangang can climb trees is this: "Pig" and "Zhu" are homophonic, and the later Tang Dynasty was indeed destroyed by Zhu Wen, and the Tang Dynasty was also called, Li's moral is tree, which accords with the answer that pigs can climb trees ~