A spring is a mechanical device that exhibits elasticity according to Hooke's law. Most springs are made of steel, brass, or bronze. The commonest type is the helical spring, a helical coil of stiff wire, loose-wound if to be compressed, tight-wound if to be extended under tension. Springs have many uses, including closing valves,spring balances, and accelerometers. The spiral spring is a wire or strip coiled in one plane, responding to torque applied at its inner end, and used to store energy, notable in clocks and watches. The leaf spring, used in vehicle suspension systems, consists of several steel strips of different lengths clamped on top of each other at one end. When deformed, springs store potential energy, and exert a restoring force. Hydraulic and air springs work by compression of a fluid in a cylinder.