This may be a simple question for some of you but it has baffled me for a long time. When we say that spacetime is curved, do we mean that from a flat space of a higher dimension, our spacetime would appear curved, in the same way that the surface of a sphere looks curved when viewed from flat 3-space? And if so, then what is the stuff of this spacetime? In the case of the sphere, we know that the surface is made of some material which can be curved. But in spacetime, what is it that is curved? Is the curvature only observable through the effects it has on the motion of photons and the like? Or do we really mean that clocks run at different rates in the field and that rods measure dfferent lengths in the field and we call that curvature? And a related question that is bugging me is: Do we know of any mechanism by which matter/energy/pressure actually causes the curvature - besides the recipe for calculating that using Einstein's field equations? What I am trying to understand is - how does matter and energy interact with spacetime causing it to bend? I suppose I am getting to the following - is spacetime actually flat but endowed with a gravitational field which effects clocks and rods in such a way as to give us the impression that spacetime is curved? Thanks a lot, Jimbob.