A coffee cup built with a continuous inside/outside surface as a references to a Klein Bottle | buy it here
In mathematics, the Klein bottle ( /ˈklaɪn/) is a non-orientable surface, informally, a surface (a two-dimensional manifold) in which notions of left and right cannot be consistently defined. Other related non-orientable objects include the Möbius strip and the real projective plane. Whereas a Möbius strip is a surface with boundary, a Klein bottle has no boundary. (For comparison, a sphere is an orientable surface with no boundary.)
