The most difficult crossword puzzle in the world is best explained.

Answer: Day.

Commentary: Zhang Qinan, a puzzle solver in the Qing Dynasty, cited an example in "Talk about Spring Lights in the Garden": "Leaves are like the foam of a waterfall, falling one after another" and typed the word "day". Those who don't know the password will not know the cause and effect when they uncover the mystery. This mystery was first associated with the names of emperors Xiao Daocheng and Xiao Yan in the Five Dynasties and the Qi and Liang Dynasties. In the Qi and Liang Dynasties, it was given a "Chen" (traditional), and then "boundless, falling wood" (that is, going to "Lian" and "falling wood") became "heaven". Because of the strange password used in this riddle, Mr. Zhang Qinan called it "incredible" and "the deepest mystery".