Classic 15-puzzle (sliding puzzle) — slide numbered tiles into the empty space to sort them from 1 to 15.
The board has 15 numbered tiles and 1 empty space.
Tap a tile adjacent to the empty space to slide it in.
Goal: arrange 1 to 15 in order, with the empty space at the bottom-right.
The 15-puzzle is solved in layers, not all at once. Lock in the top row first (1, 2, 3, 4) and then the left column (5, 9, 13), and once those are in place never disturb them again — every later move happens in the shrinking area below and to the right. Trying to place tiles in number order all over the board is how people get stuck shuffling forever.
The fiddly part is the end of each row, like fitting 3 and 4 together in the corner. The standard trick is to park them just below their target, then rotate them into place as a pair. Tiles already correct should be treated as walls you steer around. With practice the bottom 2×3 becomes a familiar rotation you can solve almost on autopilot.