We just learned about the Rotor.
Another part of a car is the Brake Caliper.
We know the rotor is a disc shaped part inside the wheel that is spinning around.
The caliper goes on the outside of the rotor, and when you hit the brakes it pushes onto the rotor to stop it from spinning around.
(from: wikipedia - disc brake)
Kid Facts - Blast from the past: Valve Gear