From our "intrinsic" point-of-view on the surface of the Earth, it appears to be flat, but if we examine the Earth from the "extrinsic" point of view, somewhere off the Earth's surface, we can see that it is clearly a curved surface. Amazingly, it is possible to determine that the Earth is spherical simply by taking measurements on its surface, and it is possible to generalize these measurements in order to study the shape of the universe. Mathematicians such as Riemann did just this, and Einstein was able to apply these geometric ideas to his "general theory of relativity", which describes the relation between gravitation, space, and time.