Ben Wa balls, also known as Burmese bells or Geisha balls (In Chinese language|Chinese, Ben Wa balls are called 阴道球 -Pinyin:yīndàoqiú, vagina ball- or 内用球 -Pinyin:nèiyòngqiú, internal-use ball-) are small, marble-sized metal balls, usually hollow and containing a small weight that rolls around—used for sexual stimulation (by insertion into the vagina or anus). Some may also be solid, or contain clappers or chimes within; other larger versions made of plastic encasing lesser balls are called Duotone balls. Reportedly, there is also a pair consisting of one containing a drop of mercury and the other, a tiny vibrating metal tongue, giving a uniquely erotic sensation even at the slightest movement of hips or legs (Sex in History, Reay Tannahill). They are used by inserting them into the vagina or anus and using muscles to hold them in, and movement to stimulate and vibrate.
Ben-Wa Balls have a varied history, their origin and method of construction changing drastically depending on where you look. Most information concerning Ben-Wa Balls is vague and probably apocryphal. It is said that they originally were a single ball placed in the vagina, used to enhance the act of coitus, but shortly evolved into multiple metal covered balls linked by either a chain or silk string for easy removal. Usage of Ben Wa balls creates a subtle stimulation, not meant to bring the user to immediate orgasm but rather to tease. It is possible to leave Ben Wa balls in one's vagina all day, or use them while seated in a rocking chair, for an effect that some find pleasurable.
At the end of the sixteenth century, Ralph Fitch, an English traveller, encountered them in the Shan States of Myanmar. He described their use there by men. Less than a dozen years later, the Florentine merchant Francesco Carletti also reported their use in Thailand, where they were used to enlarge the penis.
Ben Wa balls may also be used to simply increase the strength of the pelvic floor muscles, much as the Taoist practice The Deer Exercise has been used for centuries, and the Kegel exercises are used today. Similar to Kegel exercises, Ben Wa balls and other shaped vaginal weights are recommended by gynecologists and obstetricians to increase vaginal elasticity and bladder control. Vaginal weights come in a cylindrical shape and are used in increasing weights to strengthen the vagina, which improves sexual performance. Mild urinary incontinence, such as that which occurs while coughing, laughing, or sneezing and occurs more often as women grow older, can be treated with Ben Wa balls, vaginal weights, or Kegel exercises.
The effect of Ben Wa Balls as erotic stimulator is disputed due to the fact that sexologists claim either no or very little sensation is felt by a woman. However, practitioners of such spiritual traditions as Tantra and Chinese Taoism believe that Ben Wa Balls are helpful tools to exercise control and to explore the sensual side of an individual. 
They are also used in the postnatal period.