Fifteen Puzzle — free browser game

Your goal is to arrange the scrambled numbered tiles in ascending order from top-left to bottom-right and bring the empty cell into the bottom-right corner. The default 4×4 board has tiles numbered 1 to 15 plus a single gap; using the buttons at the top, you can also switch to a 3×3 (easy) or 5×5 (hard) size. To make a move, click a tile that is horizontally or vertically adjacent to the gap — the tile slides into the gap and the gap takes the tile's place; there is no diagonal movement or selecting distant tiles. On the keyboard, the arrow keys move the gap (the tile underneath slides in the opposite direction), and the R key reshuffles the board. The timer and move counter start with your first move; your record, consisting of the fewest moves and the shortest time, is stored separately for each size. Strategy: lock down the top row and the left column in order and then work in the remaining smaller area; when placing a corner, the classic deadlock is avoided by first aligning the last two tiles in reverse order and then rotating them into place.