Imagine a big sheet of flexible, stretchable fabric like rubber or spandex. This sheet is like a two-dimensional surface, which can be dimpled, bent, twisted, or poked, depending on what objects are placed on it. Spacetime can be thought of as a flexible, bendable structure just like this rubber sheet, except that it is four-dimensional and its lengths and distances are related mathematically by the Friedmann-Robertson-Walker metric.