Imagine that the presidents of Forwardland and Backwardland are again on opposite ends of a moving train, but the peace treaty has collapsed. This time they are dueling with laser guns. (President Forwardland is facing forward, in the direction of the train’s motion, and President Backwardland is facing backward.) Each is set to fire his gun when light from the bulb reaches his eyes. According to those on the train, the light reaches each president at the same moment and they fire their guns at the same moment.