Inner tubes are torus-shaped balloons made from an impermeable material, such as soft, elastic synthetic rubber, to prevent air leakage. The inner tubes are inserted into the tire and inflated to retain air pressure. The valve stem is a tube made of metal or rubber, through which the tire is inflated, with a check valve, typically a Schrader valve on automobiles and most bicycle tires, or a Presta valve on high-performance bicycles. Valve stems usually protrude through the wheel for easy access. They mount directly to the rim, in the case of tubeless tires, or are an integral part of the inner tube.