The surface of an ordinary sphere, like the Earth, is called a two sphere, or <math>\mathbb{S}^2 \ </math>. The defining quality of a sphere is that it is the set of all points in a Euclidean (i.e., normal) three dimensional space which are the same distance from a set point. Note that the interior of a sphere is not <math>\mathbb{S}^2 \ </math>, and that like <math>\mathbb{S}^1 \ </math>, a deflated beach ball stretched into some strange shape is still <math>\mathbb{S}^2 \ </math>, so long as it does not self-intersect. | The surface of an ordinary sphere, like the Earth, is called a two sphere, or <math>\mathbb{S}^2 \ </math>. The defining quality of a sphere is that it is the set of all points in a Euclidean (i.e., normal) three dimensional space which are the same distance from a set point. Note that the interior of a sphere is not <math>\mathbb{S}^2 \ </math>, and that like <math>\mathbb{S}^1 \ </math>, a deflated beach ball stretched into some strange shape is still <math>\mathbb{S}^2 \ </math>, so long as it does not self-intersect. |