Slide tiles into the empty space to put every number back in order.
Tap any tile next to the empty space to slide it into that gap. Keep sliding tiles until every number sits in order, reading left to right, top to bottom, with the empty space in the bottom-right corner. Switch between the easier 3x3 (8-puzzle) and the classic 4x4 (15-puzzle) using the tabs above the board.
Work on one row at a time starting from the top — once the first row is correctly placed, avoid disturbing it while solving the rest. Solve the last two tiles of a row together with a small rotating maneuver rather than trying to place the second-to-last tile directly, since placing it directly often displaces the one before it. Save the bottom-right corner for last, since that's where the empty space needs to end up.
Is every shuffle solvable? Yes — puzzles are shuffled by making random valid slides from the solved state, which guarantees a solution always exists.
What's the difference between the two sizes? The 3x3 board (8 numbered tiles) is a faster warm-up; the 4x4 board (15 numbered tiles) is the classic, more challenging version.
Does my best move count reset when I switch board size? No — each board size tracks its own separate best move count.