Hi! I was havin again one of my "weaker moments" as we were trawelling to our wacation at a freeway and numerous cars were speeding to pass our car. I begun to think about speed differences like 120 km/h compared to 60 km/h is of course 60 km/h. And 0 km/h compared to 60 km/h is also 60 km/h. But wait - 0 km/h mans you're not moving at all... What a heck - please explain. Thanks!