A ten-pin bowling ball and two pins. A bowling ball is a round ball made from rubber, urethane, plastic, isinglass, reactive resin (solid, particle, or pearl), or a combination of these materials which is used in the sport of bowling. Ten-pin bowling balls generally have a set of three holes drilled in them, one each for the ring and middle finger, and one for the thumb; however, rules allow for up to five finger holes. A five-pin bowling ball has no finger holes and is smaller so that the bowler can hold the ball in the palm of their hand. Candlepin bowling balls also fit in the hand, but are lighter than five-pin balls. Most bowling alleys provide free balls for patrons to use, called house balls, although avid bowlers may purchase their own. These are often customized, and can feature specially sized finger holes (in the case of ten-pin balls) or monograms. Because purchased balls are usually drilled to match the owner's fingers, most can throw a customized bowling ball that is one to two pounds heavier than the house ball they previously used. Bowling balls come in many varieties of colors, and are often either a single flat color, a swirl-like design of multiple colors, or a single color with an iridescent look. It is even possible to obtain transparent bowling balls, painted in such a way as to make it appear as though an object is inside. Some objects have included skulls, footballs, and baseballs. Inside the ball is a core which makes each ball do something different. These cores are dynamically imbalanced to cause the ball to try to stabilize as it is rolling down the lane.[1] This makes the ball roll over a different point on the surface every time it revolves and "flares" or causes multiple oil rings. Bowling ball cores are constructed with different densities as well. Some are center heavy (low rg) and some cores are made with the weight distributed more towards the cover of the ball (high rg). Low rg balls will spin more easily than the high rg balls.[2]