In order to function, the microprocessor requires inputs from sensors. These inputs are used to make decisions which, in turn, are used to make outputs that enable the operation of various actuators. Many of these sensors and actuators operate on electronic principles. Materials such as copper have many free electrons in their outer shells. These free electrons can be made to flow in a circuit under the influence of an applied voltage, and they are good conductors. Semiconductor materials such as pure silicon have few electrons in their outer shells, which makes them poor conductors of electricity. When electrons have been moved into a larger orbit they may fall back to their original energy level. When this happens, the electrons give up energy in the form of heat, light, and other forms of radiation.