A clock is made by putting a light output at the bottom of a long pole, a mirror on top of the pole, and an electronic light detector down at the bottom of the pole. The clock is started by briefly closing a switch that sends one blink of light from the bottom of the pole to the top of the pole where it is reflected back to the bottom of the pole. When the light detector at the bottom of the pole sees the blink of light it does two things. It adds one to the counter that is attached to it, and it fires another blink of light up to the mirror. When that blink of light gets back to the bottom, the count changes to two, and another blink of light is set off. Since light travels very rapidly (300,000 kilometers/second), for each second measured by an ordinary clock the light clock will "tick" a very large number of times. 