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. AThe set of Nova balls have three gradual increased weights for different exercise needs. Effective kegel exercise helps to strengthen a woman' s PC muscle (pubococcygeus 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.Specifications: Material: Body-safe silicone, Single Ball Dimension: 36X135 mm, Double Ball Dimension: 32X170 mm, Double Ball Dimension:28X162 mm, Weight: 49g, 75g, 95g, Water Repellency: 100% waterproof. From Svakom.