Brake pads and shoes can be made of ceramic, metal or organic materials, while the disc rotors and drums they press against are made of metal. Because the pads and shoes create friction to stop the car, they gradually wear down over time and may wear away completely, letting the metal of the calipers and cylinders they are attached to grind against the rotors and drums and damage them. (Some pads have a metal strip attached that sounds a warning whistle when the pad becomes too worn, but this strip sounds only when the car is in motion and the brakes are not applied.