Mathematical
Möbius Strip
A surface with only one side and one edge
Overview
A non-orientable surface formed by giving a strip a half-twist before joining the ends.
A Möbius strip has only one surface - you can draw a continuous line covering the entire 'surface' without lifting your pen or crossing an edge. It also has only one boundary edge. An ant walking on it would traverse 'both sides' before returning to start.