Hobby, I heard that hand-cranked iron balls are also very useful. This kind of iron ball is commonly known as fitness ball. It is not only an entertainment tool, but also has the function of public watching fitness. When it is held in your hand, it has different sound effects, high and low, crisp and pleasant, which can not only adjust your muscles and mind, but also cultivate your temperament in the lingering and pleasant sound of iron balls. In the old days, bureaucrats and businessmen who traveled to and from Yuzhong, Baoding, Beijing, all wanted to buy this as a gift for relatives and friends or dignitaries.