I understand that the curvature is caused by the depression of a mass in the space-time surface. What I don't understand is what is causing this depression. For example, is it the bodie's resistance to the motion of the space-time surface? Or is it that the curvature is caused by the displaced volume of empty space (like a marble travelling through water)? Further, I don't understand how you can curve something that has no substance (ie. empty space). Also (this may sound dumb), assuming that the space-time surface is two-dimensional, can a body move parallel to this surface?