15 puzzle
Tap a tile and it slides toward the gap; tiles further along the same line come with it. Arrow keys work as well. Every board you are given can be solved — a freely shuffled board is unsolvable half the time, so the shuffle checks and repairs itself before handing the board over.
Tap a tile to slide it toward the gap. Tiles in the same line slide together. Arrow keys work too.
Lock the top row first and leave only a 2×2 at the end
Put 1, 2, 3 and 4 into the top row first and never touch that row again. Next is the left column: 5, 9 and 13. Fixing one line at a time leaves a 2×3 block in the bottom right of the 15-puzzle, and once 6, 7, 10 and 11 are in, only a 2×2 remains. A 2×2 is just the gap going round in a circle, so it ends within a few moves. The hard spot is the last two tiles of a line — park 3 in 4's place, pull 4 in underneath it, then rotate both in together.
Arrow keys move the tile, not the gap
Press the left arrow and the tile to the right of the gap comes left. Some versions read it as the gap moving; here it matches the direction you would push with a finger. WASD works the same way. An arrow key moves one tile at a time, while tapping a tile slides everything between it and the gap in one go. Either way, tiles moved are what is counted.
Records are kept per size, and today's board is kept apart
The best is separate for the 8-puzzle and the 15-puzzle, and it changes only when a solved board took fewer tile moves than the stored value. Tying it does not count as beating it. Today's board is shuffled from the date, so everyone gets the same board on the same day; it is always the 15-puzzle and changes at midnight Korean time. Only your first attempt of the day is written down, and it never mixes into the best — replaying the same board until a good number appears would make the scoreboard say nothing.
What this screen does not do
No timer; only tiles moved are counted. There is no undo, and the board is not stored, so a refresh shuffles a fresh one. No hints, no solver. The record stays in this browser only.
Common questions
QI heard some boards can never be solved.
True. Lay sixteen tiles out at random and half the results are impossible — across 20,000 shuffles, 49.7% were solvable. So this page checks after shuffling and, if the board is unsolvable, swaps two tiles to repair it. You are never handed an impossible board.
QHow can you tell whether a board is solvable?
Count the pairs that sit out of order. On an odd-width board it is solvable when that count is even; on an even-width board, when the count plus the gap's row from the bottom is odd. Rather than trust the remembered rule, all 362,880 arrangements of the 8-puzzle were generated and compared against the boards actually reachable by sliding — not one disagreed.
QWhat is the hardest board worth?
For the 8-puzzle, 31 moves. That comes from walking outward from the solved board through all 181,440 reachable positions; the average is 22. The 15-puzzle has far too many positions to sweep that way, so no figure is claimed for it here.
QWhat does the move count measure?
Tiles moved, not taps. Sliding a distant tile in the same line drags the tiles between it and the gap, and each of those counts — otherwise a three-tile push would be free. Only solved boards set a record, kept in this browser and never sent to a server.
QShould I start with the 8-puzzle or the 15-puzzle?
The 8-puzzle. On 3×3, once the top row is fixed only a 2×3 is left, so the fix-one-line-at-a-time habit forms within a few games. The 15-puzzle simply has one more line. No 8-puzzle board needs more than 31 moves.
QDoes starting a new game erase my record?
No. It only reshuffles the board and resets the move count to zero. The best stays. On today's board, a new game brings back the same layout.