There is no junior high school that doesn't take the Hu Bugui exam. There is no special Hu Bugui model in junior high school, but it permeates some questions; Hu Bugui's model comes from a tragic story: once upon a time, a teenager went out to study. One day, he was unfortunately told that his old father was critically ill, so he hurried home at once. The place where he started was a piece of sand between his home and his home. He followed the principle of "the shortest straight line between two points" and took the "shortcut" without hesitation. When he got home, he still didn't see the old man for the last time. The teenager regretted it and burst into tears. Later, he listened to his neighbors and kept saying, "Hu Bugui, Hu Bugui, what do you mean, why haven't you come back?" As a result, this tragic story was later interpreted as a mathematical question.