In the seventies of the 19th century, Noyes Palmer Chapman, who worked as a postmaster in 'normal life', invented a single-player game which caused some sensation. The puzzle author and chess player Sam Loyd claimed that he was responsible for its invention or its popularity, too, but that was untrue. The 14-15-Puzzle (also called 15-Puzzle, Game of Fifteen, Gem Puzzle, Mystic Square...) consists of a grid of 4x4 parts with 15 square pieces. Their arrangement can be varied by moving a tile next to the missing piece into the empty space.
The pieces are numbered accurately from 1 to 15, honestly spoken, almost accurately: 14 and 15 are exchanged.
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It's the player's task to arrange a proper order from 1 to 15, without exception.
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A prize of 1,000 $ was promised to whom who would solve the problem at first. There are a lot of rumours about people who thought they did it but unfortunately brought the tiles into disorder again... However, the prize never was never demanded. - A lot of difficult riddles start off simple, nevertheless: If thousands of people couldn't find a solution though searching hard, it is possible that there is none. This is, of course, no mathematical prove, but it maybe is more prosperous asking why nobody found a solution than searching for one.
