The 15 Puzzle is a classic sliding-tile puzzle where scrambled tiles need to set in the proper order. The tiles are set in a 4x4 grid with one position empty, and tiles can only be moved by sliding into the empty space. It is a one-player, perfect-information game: the player always knows the exact position of tiles and possible moves. This makes it a useful test of search algorithms, which automatically play out various moves and evaluate the result to find the best sequence of moves.