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Flexagons are flat models made from folded strips of paper that can be made, by folding or flexing, to reveal a number of hidden faces. They are amusing toys but have also caught the interest of mathematicians.

Flexagons are usually square or rectangular (tetraflexagons) or hexagonal (hexaflexagons). A prefix can be added to the name to indicate the number of faces that the model can display, including the two faces (back and front) that are visible before flexing. For example, a hexaflexagon with a total of six faces is called a hexahexaflexagon.

The discovery of the first flexagon, a trihexaflexagon, is credited to the British student Arthur H. Stone who was studying at Princeton University in the USA in 1939. Stone's colleagues Bryant Tuckerman, Richard P. Feynman and John W. Tukey became interested in the idea. Tuckerman worked out a topological method, called the Tuckerman traverse, for revealing all the faces of a flexagon. Tukey and Feynman developed a complete mathematical theory that has not been published.

Flexagons were introduced to the general public by the recreational mathematician Martin Gardner writing in Scientific American magazine.