Ben Wa balls, also known as Burmese bells or Geisha balls (In Chinese, Ben Wa balls are calle 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.