I have a quick question regarding causality in SR and some specific examples. Collision between two bodies is one of the examples when one event causes another, let's say that a body travels towards another which is at rest and hits it. Now my question may seem strange, but what is really the cause of the acceleration of the second body after the hit. Does the first body as a whole cause the second one to move, or can we consider only the surface of the first body the cause of the event 'the second body starts to move', since only the surface had the interaction with it and therefore we are speaking of two time-like events? But this still sounds confusing since the body as a whole has its mass and force and it seems that when speaking in those terms, we should somehow consider that the body as a whole had something to do with the start of the motion of the second body. I appreciate your help, thanks in advance.