The Möbius strip is an example of a surface that has only one side and one face.
This can be verified by twisting one end of a paper strip 180° and attaching it to the other end.
Colouring the outside of the strip will emphasize the fact that the strip no longer has two sides, but only one.
A line drawn along the strip will lead back to the starting point without lifting the pencil from the paper.
August Ferdinand Möbius (1790-1868) was a German astronomer and mathematician.