A friend of mine is an engineer. Let's call him Sam. Somewhere in the bowels of Sam's current work place, is on old steam engine. Obviously, this steam engine is pretty heavy, it is not made of feathers and it is not easy to move. I know, because Sam tried. He pushed it with all the force he could muster but...no movement. Then, one day, Sam decided to lean against the train for a while and...lo and behold...it began to roll.