How did the ancients say "white clouds and pale dogs" come from?

White dog means that the clouds in the sky are like white clothes, and they soon become black dogs, which are used by people to describe impermanence. Here is an allusion of Du Fu.

Du Fu, a great poet in the Tang Dynasty, was helpful, but often had no food and clothing, so he had to use poetry to help others. Li Guinian was an artist and a famous musician in the Kaiyuan period, and was deeply loved by Li Longji, Emperor Xuanzong of the Tang Dynasty. He is often invited by princes to play and sing, and he always gets many awards.

After the Anshi Rebellion, Li Guinian was forced to live in the south of the Yangtze River, and his life was very poor. At that time, Du Fu happened to be in exile in the south. At a banquet, Du Fu heard Li Guinian's singing again, and he was very emotional when he thought of their situation. He wrote a poem to praise Li Guinian's superb art and expressed his feelings: It is very common in the house of the King of Qi, and Cui has heard it several times before. Never thought, in this Jiangnan scenic area; It is the season of falling flowers, and I can meet you, an old acquaintance.

It means: "I didn't expect to meet your old friends again in this beautiful south of the Yangtze River and in this colorful season of losing English." Look at the beautiful scenery in the south of the Yangtze River and think about our sincere friendship. Maybe a happy meeting can make us forget the sorrow. "

There was a poet named Wang at that time, and his family was very poor. He makes a living by selling straw sandals. He often eats the last meal without the next, but this man lives in poverty and likes reading and writing poems in his spare time. His wife couldn't stand the poor life and left him to a rich man. Others laughed at him and gossiped about him.

Du Fu couldn't help him, so he wrote a poem to comfort the king. "Clouds in the sky are like white clothes, and they must become like pale dogs", which means "Clouds in the sky are like white clothes and will soon become black dogs. Throughout the ages, things have changed. What can't happen in life? " Wang was very grateful after reading it.