Sphereing, also known as globe-riding and Zorbing, is a recreational activity in which one, two or three people ride inside the inner wall of a plastic ball that rolls on the ground. To cushion the rider from bumps, there is about half a meter (about two feet) of air between the inner and outer walls of the ball. Some balls have straps to hold the rider(s) in place and some leave the rider(s) free so that they can influence the ball's movement or be tossed around freely by the ball's rolling and bouncing, optionally with a sloshing amount of water inside. People concerned about their balance, safety or other needs may enjoy the activity on ground that is either level or only slightly rolling.