(Euclidean geometry deals with non-curved surfaces).Imagine space as a two dimensional structure -- a Euclidian universe would look like a flat plane.Parallel lines are only possible on a flat plane.

• 零曲率在一个平面上或者欧几里德空间(欧几里德几何学专门用来处理非曲面)上描述宇宙，一个欧几里的空间就是一个平面，在一个平面内才有可能出现平行线。