Sliding Puzzle

The sliding puzzle — most famously the 15-Puzzle — is a mechanical puzzle where 15 numbered tiles sit in a 4×4 frame with one empty square. Tiles slide only into the empty square; the goal is to arrange them in ascending order. Invented in 1874 by Noyes Palmer Chapman, it sparked a global craze in 1880 comparable to the later Rubik's Cube boom. Half of the starting positions are provably unsolvable — a mathematical fact that fuelled Sam Loyd's famous $1,000 challenge.