Energy can exist in different forms and can change from one form to another. There are six main types of energy: light, sound, heat, electrical, mechanical, and chemical. Energy can also be categorized as either kinetic energy, which is energy in motion, or potential energy, which is stored energy. A simple machine is a device with few parts that makes work easier by changing the direction or magnitude of force. There are six basic types of simple machines: lever, wheel and axle, inclined plane, pulley, screw, and wedge. Friction opposes the motion of simple machines but can be reduced by wheels, oil, or grease. A compound machine is a device made of two or more simple