[Game of Fifteen][1], a.k.a. the 15-Puzzle, was very popular 100+ years ago. The game is still found in stores; for example, you can search Amazon for "15 Puzzle" and get it for about 5 dollars. In the paper [Notes on the 15 puzzle][2] (American Journal of Mathematics, 1879, Vol. 2, No. 4, 397-404) W.W. Johnson and W.E. Story showed, via parity argument, that half of the positions in the puzzle [are not solvable][3]. The parity argument is accessible even to high school students. Editors' note accompanying the paper by Johnson & Story (1879) in American Journal of Mathematics is quite interesting: > The "15" puzzle for the last few weeks has been prominently before > the American public, and may safely be said to have engaged the > attention of nine out of ten persons of both sexes and of all ages and > conditions of the community. But this would not have weighed with the > editors to induce them to insert articles upon such a subject in the > American Journal of Mathematics, but for the fact that the principle > of the game has its root in what all mathematicians of the present > day are aware constitutes the most subtle and characteristic > conception of modern algebra, viz: the law of dichotomy applicable to > the separation of the terms of every complete system of permutations > into two natural and indefeasible groups, a law of the inner world of > thought, which may be said to prefigure the polar relation of left > and right-handed screws, or of objects in space and their reflexions > in a mirror. Accordingly the editors have thought that they would be > doing no disservice to their science, but rather promoting its > interests by exhibiting this *a priori* polar law under a concrete > form, through the medium of a game which has taken so strong a hold > upon the thought of the country that it may almost be said to have > risen to the importance of a national institution. Whoever has made > himself master of it may fairly be said to have taken his first lesson > in the theory of determinants. It may be mentioned as a parallel case > that Sir William Rowan Hamilton invented, and Jacques & Co., the > purveyors of toys and conjuring tricks, in London (from whom it may > possibly still be procured), sold a game called the "Eikosion" game, > for illustrating certain consequences of the method of quaternions. > -EDS. [1]: http://en.wikipedia.org/wiki/15_puzzle [2]: https://www.jstor.org/stable/2369492 [3]: https://en.wikipedia.org/wiki/15_puzzle#Solvability