The moon in the vast sea has tears, and the next sentence is, swallows return, oh, joy is endless. This was a famous line from the Tang Dynasty poet Zhang Ruoxu's Spring River Flower Moonlit Night. It meant that on a moonlit night by the river, a pearl by the beautiful spring river glistened with tears, making one sad. And the next line was " swallows return ", which meant that the music was still playing. This sentence depicted a scene full of sadness and longing, but it also revealed a kind of hope and beautiful emotions.