The sliding puzzle is a board of numbered tiles with one empty square. Slide tiles into the gap until they read 1, 2, 3… from left to right and top to bottom, with the gap in the bottom-right corner. It looks random at first, but a steady method solves every board.
How to play
Tap a tile in the same row or column as the gap. It slides toward the gap, along with every tile in between.
Swipe on the board, or use the arrow keys or W, A, S, D, to move the tile next to the gap in that direction.
Each tile that moves counts as one move. Undo takes back your last slide.
The puzzle is solved when every tile is in order and the gap is in the last corner.
Every puzzle can be solved
Only half of all tile arrangements can be slid back into order. Swap any two tiles on a solved board and it can never be solved again. Every board here is shuffled so it can be solved, so if you are stuck, it is the method, not the deal.
Solve it one row at a time
Put the top row in place first, then leave it alone for the rest of the puzzle.
For the last tile of a row, park it under its home square, line up the tile before it, then rotate both into place together.
Carry on row by row until two rows are left. On a 3×3 board that is right after the first row.
The tile colors show which row each tile belongs in, so you can spot the next ones quickly.
Finish the last two rows by columns
With two rows left, work from left to right one column at a time. Bring the top tile of a column into place, tuck the bottom tile in beside it, and rotate the pair into the column. The final three tiles then cycle around the gap until they line up.
Boards, time and moves
3×3 (eight tiles) is a quick warm-up that usually takes under a minute once you know the method.
4×4 is the classic 15 puzzle.
5×5 has 24 tiles and takes patience, but uses exactly the same method.
Your best time and fewest moves are kept separately for each board. Sliding a whole row at once saves time, but every tile still counts as a move.