Slide the numbered tiles into order using the one empty space, in as few moves as possible.
The grid is scrambled with one square missing. Tiles next to the gap can slide into it. Keep sliding to arrange every tile in numerical order, ending with the blank in the corner. Choose your grid size, from the 3x3 eight-tile board up to the larger 24-tile board.
The 15 puzzle was devised by Noyes Palmer Chapman, a postmaster in Canastota, New York, who was showing friends a precursor as early as 1874 — sixteen numbered blocks to be arranged in rows of four, each row summing to 34, which makes its true ancestor the magic square.
It spread by hand rather than by publisher. Copies travelled through Chapman's son to Syracuse, then to Watch Hill in Rhode Island, and finally to Hartford, where students at the American School for the Deaf began manufacturing them; by December 1879 they were selling locally and in Boston.
The famous story is false. Sam Loyd claimed from 1891 until his death in 1911 that he had invented the puzzle — his first article on it was 1886, five years before the claim, and he had no connection to either its invention or its initial popularity. He did fuel a later craze by offering $1,000 for reversing the 14 and 15, a configuration that cannot be reached from a solved board no matter how long you slide.
Fifteen is the traditional sliding-tile number puzzle, free to play in your browser across several grid sizes. Built in-house at PlayPendium.