Someone help me figure out this couplet, a master.

The sun and the moon are bright on three days and crystals appear

Shi Naian is the author of "Water Margin", one of the four famous works in my country, and is a household name. But what few people know is that he is also a doctor with superb medical skills. Shi Naian once lived in Xinghua County, Jiangsu Province. At that time, there was a patient named Gu Fei. He had been ill for several months and was in a daze. He kept chanting "Sunny day in May..." all day long. Such a strange disease made Shi Naian An also felt that it was difficult to prescribe the right medicine for the time being.

One day, he sat in front of Gu Fei's bed and carefully observed the patient's condition and changes in demeanor. When the patient said to himself "It's a sunny day in May", he immediately said "San Chunfang" grassland". At this time, the patient's illness was gone. He opened the quilt, got up and got out of bed. He solemnly saluted Shi Naian and said, "The ancient trees in front of the rocks and rocks have dried up and this wood is used as firewood."

Shi Luo thought about it and said : "The sun and moon in the white water spring are crystals for three days."

At this point, Gu Fei recovered from his illness. It turned out that Gu Fei was suffering from lovesickness, and a girl he admired was a talented woman. The girl asked for two pairs of couplets before agreeing to marry him. He thought hard. The more anxious you are, the less you can think of a good couplet to match it, so you suddenly become ill. With his profound literary knowledge and superb medical skills, Shi Naian saved the long-term patient with half the effort and created a happy couple.

(According to "Humor and Jokes" Deng Zhu/text)