Air flow balls are small, plastic balls, often used in physical training because of their many gaping holes, which help reduce backspin and increase friction, thus hampering the ability to travel long distances. Also, the light plastic shell cuts down on damage to other objects when the ball hits them, making them good for recreation in urban and suburban areas. Because they have very thin plastic shells, they break easily. Also their low mass lets them be carried by the wind, making them potentially irretrievable.