Daily 15 Puzzle
The goal
Get the tiles into order, 1 to 15, with the gap at the end.
Click any tile in the same row or column as the gap and the whole run slides at once. Arrow keys work too, and they move a tile in the direction you press.
What par means
The number next to your move count is the fewest moves this particular scramble can possibly be solved in. It is not an estimate. Each day's board was searched exhaustively before it was published, so beating it is impossible and matching it is a real result.
Controls
Progress and your timer save in this browser as you go, so you can come back to the same board later today.
Half of All Scrambles Cannot Be Solved
Take a solved board, pop two tiles out and swap them. That position looks ordinary and it is completely impossible. Every legal slide changes the arrangement in a way that preserves a hidden quantity, so the sixteen tiles split into two separate worlds and no amount of sliding crosses between them. Exactly half of all arrangements can reach the solved board. The other half never can.
This is why the scrambles here are never generated by shuffling the tiles at random. Each one is made by sliding outward from the solved board, which keeps it in the reachable half by construction.
Sam Loyd knew it. He offered a thousand dollar prize for solving a board that had the 14 and 15 swapped, which is exactly the impossible case, and he never had to pay. He also claimed to have invented the puzzle, which he had not. That was Noyes Chapman, around 1874.
How Hard Can It Get
The hardest possible 15 puzzle board needs 80 moves. Today's is deliberately gentler than that, somewhere in the low thirties, which is enough to need a plan but not so much that it turns into a chore.
The usual approach is to finish the top row, then the second row, then solve the bottom six as a block. Resist fixing the last two tiles of a row one at a time, because placing one knocks the other out. Place them together as a pair instead.
More to Play
There are sixteen daily sudoku variants, a nonogram with hand drawn pictures and 2048. If you would rather hold the real thing, the puzzle tools page has solvers for physical puzzles, and there are plenty in mechanical puzzles.