If you hear a sound like that of an over-grown rat， gnawing between the walls or above you， stop！ This distinct， irritating sound is probably not animal-produced. In China， you will perhaps be hearing the grating， rasping sounds from human hands manipulating small objects for finger exercise. The use of objects demanding finger dexterity has long been associated with the development and maintenance of mental acuity. In fact， parents promoting the use of chopsticks as early as possible， when a child learns to feed him or herself， has been said to help the youngster’s intelligence. It is believed that skillful handling of the slender eating instruments reflects one’s intellectual ability. As we all move on in life， the challenge to maintain sharp thinking faculties remains a serious concern， and one is most likely to see the manual practice of finger exercising among senior citizens.
　　In northern Chinese provinces you may see elderly men holding two or three silver or bronze colored metal balls in one hand， absent-mindedly rotating these objects slowly producing soft clacking sounds. These balls， about the size of billiard balls， also may be wooden， black， or made of colorful marbled stone. However， in Beijing the use of special looking walnuts is perhaps more popular for these hand and finger exercises. The rasping， gnawing sounds are likely to be heard while you sit or stand near a passenger on the subway or a bus. In some instances， when your taxi driver is not preoccupied on a mobile phone during driving duties， he or she will even fondle those strange looking round or oblong objects while， hopefully， keeping the other hand on the steering wheel.
　　These “nuts” are called hetao， and their surfaces are usually dark and prunelike in appearance， with deep naturally rough etched lines that rise like a disorderly fresco of a wizened， extremely aged quality. The goal of the handler is to simply produce a smoother shiny surface through rubbing over time. Small hand-held objects， if not walnuts， may also be paired balls made of rock crystal， stone， glass or metal （stainless steel， bronze， or iron） with a smooth surface， and are called “mother-child’or “male-female” balls. They are manufactured in sizes suitable for all. It is said that the number of balls rotated in each hand may be gradually increased from two to as many as eight， as the user becomes more skilful.
　　In China， walnut pairs have been traditionally rotated and played with in the palm of the hand， both as a means to stimulate blood circulation and as a status symbol. Pairs of large， symmetrically shaped walnuts are valued highly and have recently been used as an investment， with some of them fetching tens of thousands of dollars. Pairs of walnuts are also sometimes sold still in their green skin， for a form of gambling known as du hetao. To a knowledgeable collector， obtaining a valued pair of identically sized objects can potentially be a good investment. 　　In Beijing you may find a variety of such special hand exercisers sold in antique markets such as the Beijing Liangma Antique Market； Panjiayuan Antique Market； or neighborhood flea markets on side streets. These products are commonly displayed on blankets spread on the ground， exhibited on old metal bed frame racks， boxed in pairs， or sandwiched among the many trinkets mobile Tibetan vendors may have stashed in their wagon loads of merchandise.
　　According to experts on traditional Chinese health remedies， hand therapy is regarded as a reliable method for curing diseases ranging from heart palpitations， abdominal pains， hiccups， dysentery， anorexia， general weakness， the common cold， and more. While specific fingers like the thumb， forefinger， and fourth finger may directly target legs and chest areas， pressing， forcibly rubbing， or kneading “acupoints” that include the palm may produce relief to troubled areas of the upper and lower abdomen. In general， massage experts claim that concentrating a nimble rotation of the round balls in one or both hands relaxes body muscles， promotes blood circulation， improves movement of joints， refreshes mental activity， and ultimately retards the aging process.
　　However， if you prefer to increase your health through the more common hand exercise of using chopsticks， manipulating chopsticks the “proper” way may be equally beneficial and less likely to draw unwanted attention. A practical purchase of inexpensive， rough-feeling walnuts for doing acupressure exercises may be something to consider beyond the obvious choice of eating walnuts for late night snacks. On the other hand， if investing in the smoother hand balls， you should be able to figure out the fake from the authentic， or you may go a little crazy trying to do so. Long ago a quotation originated saying， “The devil makes work for idle hands.” A good counter argument is that hand ball exercise may be performed along with elegant gymnastics， as a physical exercise and an enjoyment of art， beneficial for both body and mind. Thus， it may be more than mere absentminded noise making when you hear those walnuts clacking or grinding in public. Keeping sane in a rapidly changing culture is definitely more than something to just think about. By the way， to some， nuts are good stocking stuffers.