My understanding of relativity is that if two masses are in constant motion relative to each other, either one of them could be considered at rest with the other moving and it makes no difference which. However thinking about a collision between a big truck and a small car makes me wonder about this. Suppose that there is a huge 18 wheeler truck barreling down the highway toward a Volkswagen stalled in the middle of the road. Then suppose that Samantha the witch blinks out the road and everything else in the universe except the truck and the Volkswagen. When the fast-moving truck hits the Volkswagen, it will surely go flying for a considerable distance. However if we reverse the situation, and have the truck stalled in the middle of the highway and the Volkswagen striking the truck (with them being alone in the universe), the collision would have quite a different outcome, with the car merely splatting against the truck and not flying anywhere. The point here is that it does make a difference which one is at rest and which one is in motion and one cannot just arbitrarily choose one to be at rest and the other in motion. Does anyone have any comments on this? I'm confused.