A dumbbell is a metal bar supporting a weight fixed at each end, either in the form of balls or disks. It is used for exercises in gymnastics, body building, or weight lifting. The bar can vary in length, and the end-weights can be of different weight, depending on the exercise, the body-type of the person using them, and their level of expertise. A dumbbell is used to develop muscle mass. It can be used alone or in pairs.