To solve this riddle, there must be three factors: water, ship and sky, but there cannot be the word "sky", so it seems that we can only guess the four-character idiom: "A rising tide lifts a boat".
"The Yangtze River flows in the sky" is interpreted as: the river flows to the sky, and the rising tide lifts all boats.
"Sail alone, the blue sky is exhausted" is interpreted as: the ship is sailing in the blue sky, and the "ship height" is deducted.
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Yellow Crane Tower —— Meng Haoran's farewell on his way to Yangzhou
Tang Libai
My old friend resigned from the Yellow Crane Tower,
Fireworks fell in Yangzhou in March.
Lonely sails, distant shadows, blue sky,
Only the Yangtze River flows in the sky.
My friend waved goodbye to me at the Yellow Crane Tower. He will go to Yangzhou in sunny March. His sail shadow gradually disappeared into the blue sky, only to see the rolling Yangtze River rushing in the horizon.
Extended data:
The Yellow Crane Tower Farewell to Meng Haoran in Yangzhou is the work of Li Bai when he entered Sichuan to visit Zhuang. Li Bai is a poet who loves nature and likes to make friends. He is "according to a constant habit in my life". He traveled all his life in wandering and wandering, and his footprints spread all over the Central Plains, leaving many works praising the beauty of nature and friendship.
In the fifteenth year of Emperor Xuanzong of the Tang Dynasty (727), Li Baidong returned to Anlu, Hubei, at the age of 27. He lived in Anlu for ten years, but most of the time, he made friends with poetry and traveled abroad. In his own words, "it's a waste of ten years to keep wine in the land." That is, while living in Anlu, Li Bai met Meng Haoran who was twelve years older than him. Meng Haoran admired Li Bai very much and they soon became close friends. In March of the 18th year of Kaiyuan (730), Li Bai learned that Meng Haoran was going to Guangling (now Yangzhou, Jiangsu), so he sent someone to Jiangxia (now Wuchang District, Wuhan) to meet Meng Haoran. A few days later, Meng Haoran took a boat to the east, and Li Bai personally sent it to the river. When I was leaving, I wrote this song "Farewell to Meng Haoran on the way to Yangzhou by the Yellow Crane Tower".