suppose some people are on a spaceship and they want to make an accurate clock. at one end they put a mirror, and at the other end they put a simple machine. it shoots one short burst of light toward the mirror and then waits. the light hits the mirror and bounces back. when it hits a light detector on the machine, the machine says, "count = 1," it simultaneously shoots another short burst of light toward the mirror, and when that light comes back the machine says, "count = 2." they decide that a certain number of bounces will be defined as a second, and they make the machine change the seconds counter every time it has detected that number of bounces. every time it changes the seconds counter it also flashes a light out through a porthole under the machine. so somebody outside can see the light flashing every second.