Rebuild the picture. Tap a tile in the gap's row or column to slide it, or swipe the board. Arrow keys work too. Hold Peek to see the goal.
Compare a legal slide before moving. Checkmarked cells are in place. Tile distance counts grid steps from each tile to its goal; it is a lower bound, not a solution.
Previewing does not count moves. The timer keeps its normal rules. Escape closes the forecast.