The Hyperwear SandBell free weight combines strengths from some of the most popular pieces of fitness equipment. Use the Hyperwear SandBell in place of a medicine ball, slam ball, dumbbell or kettle bell and prepare to work muscles you didn’t even know existed. The active, shifting movement of the sand challenges stabilizing muscles in your core. When working out with the Hyperwear SandBell, you will engage hand- and wrist-gripping muscles that are typically not activated and strengthened. The dynamic SandBell weight fires more muscle burning more calories giving you more results in less time. Another advantage of this product lies in its safety for you and your floors. On a slick floor you can use the SandBell for a variety of ab coaster and glide or sliding core exercises. The outer sand disc shape, made of durable, stretchy neoprene, contains a leak-proof core that can be easily emptied and filled with dry play sand. Dry washed all purpose sand is recommended, which can be obtained at home improvement stores.. US Patent 8,622,877 B2. SandBell is a registered trademark.