It is described in detail in his uncle's novel "A Flower Hidden in the Sea" that the Zhang family where my little brother is located is a big family. Children at home have to receive family training from an early age, including physical fitness, kung fu and tomb-robbing knowledge. It is cruel to go to the wild (to practice grave robbery) when you are older. It is mentioned that they can learn to control their bodies and can't go to the toilet for nearly a month to facilitate underground activities.