Everybody knows that a knife is a very useful tool to have, but it is only useful when it is sharp. Otherwise you cannot really do anything with it. So imagine that you forgot to take you knife sharpener at home, but you are already underway in your vehicle. No worries! Roll your window down a little and start sharpening your knife.
It works exactly like a stone, polishing your blade in spectacular fashion. You only need a few strokes and you are good to go. It is incredible to think that the best sharpener in the world was literally next to you the whole time while you were driving.