This is turning out to be difficult for me, and I haven't even reached the part where he explains the theory of relativity itself :( "Let us suppose that the shot is fired from the guard's-van, and the echo comes from a screen on the engine. We will suppose the distance from the guard's-van to the engine to be the distance that sound can travel in a second (about one-fifth of a mile), and the speed of the train to be one-twelfth of the speed of sound (about sixty miles an hour). We now have an experiment which can be performed by the people in the train. If the train were at rest, the guard would hear the echo in two seconds; as it is, it will take two and 2/143 seconds" Isn't the motion of air molecules (inside the train) relative to the moving train no different from motion of air molecules outside the train relative to earth? Why is there a difference in speed? This got me confused about sound propagation in general, how can the compressions and rarefactions be http://bumper.gmi.edu/~drussell/Demos/waves/wavemotion.html" the place?! I would really appreciate any help.