There is fierce fighting between two countries, that we will be refered to as Country Bob, and Country Joe. To solve the problem the UN gets the leaders of Bob and Joe decide to sign a peace agreement to end the violence, the problem is that neither leader wishes to sign it first. To solve the problem they put the two leaders in a room across from each other equal distant to a light source at a long table. They decided that the peace agreement signature will occur when the light reaches each leader. Since light travels at the same velocity it reaches the leaders at the same time, and the agreement is signed and both countries are happy as there is peace between the two countries. Later two new countries, Left and Right are fighting. The US is trying to get a peace agreement signed to end the fighting, however they have the same problem as the first two countries, with the leaders of both countries having to sign the treaty at the same time. To solve the problem the leaders meet on a train, and again the same thing is set up. The two leaders sit on opposite ends of a table on the train, one facing the front of the train (F-Leader), the other facing the back of train(B-Leader). They set up the light in the middle and turn it on, and they both sign it when the light hits them. They two leaders and especially the US (finally doing something sucessful in foreign relations in the Bush admin) are quite pleased until they find out the fighting started again. It turns out to the observers outside of the train, they "saw" the F-leader sign the document just before the b-leader because the light had to travel a smaller distance even though it is at the same speed. However the people on the train maintain that the documents were signed at the exact same time. Who is correct? Why? This is meant to be a simple excerise to help explain SR, it was adapted from Brian Greene's The Elegant Universe.