Riemann Curvature??? i was watching this documentary that mentioned that riemann came up with a method to deduce whether we were on a curved surface, or on a flat surface, without leaving the surface to make the deduction. for example, for a curved 2d surface, we know it is as such as we can see it from the 3d space we are in but for a 3d curve, we need to be in the 4th dimension which is not possible. so he came up with a method to deduce this. [* by the way, what is a 3d curve?] may i know what method is this? is there anywhere i could read it on? (preferably on the internet) thanks!