Slide

Puzzle · plays in your browser, no download

Numbered tiles and one gap. Half of all arrangements cannot be solved — these are shuffled by playing real moves backwards, so every one can.

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Where it comes from

The fifteen puzzle, which was a craze in the eighteen-eighties, helped along by a famous prize for a position that was impossible to solve. That is the fact worth knowing: half of all arrangements cannot be solved at all, so a shuffled puzzle has to be built carefully or it is a cruel joke.

How to play

Put the numbers back in order, leaving the gap at the bottom right.
Moves are counted one per tile moved, not one per tap — so a slide of three costs three.
A tile that is already home turns green. On a five by five, knowing which ones are done is half the thinking.
The number at the bottom right is how far the board still is from finished: the total distance every tile has left to travel. It can only reach zero at the end.

What makes this one different

Tap any tile in the same row or column as the gap and everything between them slides, so you do not have to work one tile at a time. Half of all possible arrangements of a sliding puzzle cannot be solved — these are not shuffled at random, they are shuffled by playing real moves backwards from the finished picture, so every one of them can.

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