What's the story about Cao Xueqin's clever writing of embedded poems?

Cao Xueqin lived in Xiangshan in his later years and made a living by selling calligraphy and painting. There was a local mediocre official who came to the door in order to draw a fan and said, "My child has recently obtained a provincial examination, please draw him a picture, in a word."

Cao Xueqin has long wanted to ridicule this rich and heartless mediocrity. Now he comes to you himself, so he can't miss the opportunity. So he painted with a brush, first drawing a dead plum on the fan, and then drawing a sparrow under the dead plum. After the painting, he wrote another poem: Fan is cool, and the prince goes to school. August Mid-Autumn Festival exam champion Lang! After reading it, the mediocre officer took the poems and paintings with joy and satisfaction. After the mediocre official returned, he showed off everywhere to let others appreciate Cao Xueqin's poems and paintings. But learned people can't help laughing at everyone. Why? The mediocre official was puzzled, so he asked an old scholar to explain the mystery of this fan. Only then did he know that the poem was an embedded poem. After reading the words at the beginning of each poem, it becomes "lost turtle", and the picture means ironically "Little sparrow wants to climb high." The mediocre officer shouted that he had been cheated.