"Suppose we have two space ships [travelling opposite directions]... These people see those clocks [in the other spaceship] are spinning around too fast. These people [in the opposing spaceship] see those clocks [in the first spaceship] are spinning around too fast. After they've passed each other these people see those clocks have slowed down, and those people see those [other] clocks have slowed down. Now whose clocks have slowed down? There is no such thing as how fast a clock is going."