Each ball is equipped with a silicone string to easily take out the ball after use. The entire Nova ball is made of body safe silicone which is 100% waterproof. The set of Nova balls have three gradual increased weights for different exercise needs. Effective kegel exercise helps to strengthen a woman's pubococcygeus PC muscle, improving vaginal health while bringing sexual benefits and satisfaction. For daily exercise or postpartum recovery, Nova exercise balls are the perfect choice for your PC muscle strengthening needs.<br /><br />Specifications material body safe silicone. Single ball dimension 1.41 inches by 5.31 inches. Double ball dimension 1.25 inches by 6.69 inches. Double ball 2 dimension 1.10 inches by 6.37 inches. Weights 1.72 ounces, 2.64 ounces and 3.35 ounces. Water repellency. 100% waterproof. From Svakom.