Einstein's special theory of relativity states that the same laws of physics hold true in all inertial reference frames and that the speed of light is the same for all observers, even those moving with respect to one another. In this video segment, adapted from NOVA, one of Einstein's thought experiments is re-created to reinforce one consequence of special relativity: that events that are simultaneous to one observer are not to an observer in a different reference frame that is moving with respect to the observer in the first reference frame.