A quick note before you play
Put fifteen numbered tiles in order by sliding them into the empty square. Start when you are ready. You can restart at any time, and keyboard controls work after you select the game area. On touch screens, use the on-screen controls or tap the board. No score is uploaded, and there is no account to create.
Follow the gap
Only a tile directly above, below, left, or right of the gap can move. Finish with 1–4 in the first row, 5–8 in the second, 9–12 in the third, then 13–15 and the empty square. Arrow keys move the gap itself; selecting a tile moves that tile into the gap.
Solve in sections
Build the top row first, then work on the left side of the remaining area. Avoid disrupting a finished row while moving lower tiles into place. The opening is created by a sequence of legal slides, so it is solvable. The shuffle length is not a claim about the shortest solution. Invalid selections do not increase the move counter.
Make your own version
This game is also a free template demonstrating legal neighbor moves, permutation state, and solvable shuffling. Download its source to explore the logic, change the visuals, and experiment with a new rule.