According to the context of historical documents and poems, the word "sand" in "cold water moon cage sand" is correct. In this sentence, the word "sand" is used to describe the sand by the Qinhuai River, giving people a quiet and hazy feeling, which echoes the word "moon" behind, forming an elegant night color map by the water. At the same time, the word "sand" echoes the elements of the poem, such as "merchant girl" and "backyard flower", forming a complete poetic artistic conception.
Although "yarn" and "sand" may be difficult to distinguish in some cases because of poor handwriting and other reasons, "sand" is the correct word in this poem. Therefore, the word "sand" in "smoke cage cold water moon cage sand" should be understood as the correct word.