Durability is a primary goal. Golf balls are extremely durable and the Zippy RBT shell made from 5% Glass-filled Polypropylene is just a rugged. Because the ball is permanently sonic welded into the shell Zing Partners were able to require that Zippy RBT devices survive a six foot drop without breaking. These are tough, durable, and also capable of being used in the most gentle fashion or with great pressure equally well.