fossil fuel: A fossil is the remains of a plant or animal from a previous age preserved in rock. Fuel that is extracted from this type of rock is called fossil fuel. Petroleum, coal and natural gas are fossil fuels.
friction: When one surface rubs against another, there is a force that resists the motion. The force is called friction. Friction has two important effects. It can stop or slow down the motion and it creates heat. For example, a car that skids to a stop experiences friction between the brakes and the tires and between the bottom of the tires and the surface of the road. The friction brings the car to a stop, and it also heats up the car's tires and brakes. Even liquids and gases can cause friction.
