Manifold
The Earth is a sphere, a three dimensional object of geometry. Yet, maps (two-dimensional representations) can be made of the Earth. At the edges of a certain map, the map needs to be changed. That way it is possible to make a two dimensional image of the whole surface of the Earth. There need to be rules, on how to change the maps, and some areas (near the edges of the map) will be on more than one map. It is not possible to make one map only, which would have no edges. This map would either have edges (and overlapping areas), or there would be some places where the paper was torn. Tearing and overlapping isn't allowed.