About Sliding Tiles

One gap, a board of tiles, and the order they belong in. Tap any tile in line with the gap and it slides across, taking the tiles between with it, so a whole row or column can move in one go. The classic is the fifteen: tiles 1 to 15 on a four-by-four board, back in order with the gap in the bottom corner. It dates from the 1880s and it is still one of the best puzzles there is, because every move helps one tile and disturbs another.

Three ways to play. Numbers is the classic. Picture cuts one of the site's own photographs into squares and asks for it back. Paths gives every tile a length of pipe and asks for a line that runs from the arrow on the left edge to the arrow on the right — any arrangement that joins them wins, not only the one it was made from. Every board comes in three sizes, from the gentle three-by-three to the long, satisfying five-by-five, and every scramble is checked to be solvable: half of all possible arrangements of a sliding puzzle cannot be solved at all, and none of those ever reaches you.

On the three-by-three board the true shortest solution is known before you start, and your result is rated against it. The hint on that board is a step on a shortest path; on the bigger boards it follows the method people use by hand — the top row, then the next, then the last two rows a column at a time. The daily challenge is the classic fifteen, the same board for everyone, ranked by fewest moves with the clock breaking ties.

Tips to win

• Solve the top row first, then the one below it — never come back to a finished row
• The last two tiles of a row go in together: put the last one in the corner below where it belongs, bring the other beside it, then turn them in as a pair
• Once only two rows are left, work across them a column at a time rather than a row at a time
• Count your moves on the three-by-three against the shortest shown; it is a quick way to get much better at the big boards