Slide the tiles into order, 1 to 15.
Use the single empty square to slide tiles until 1 through 15 sit in order. Difficulty is how deeply the board is shuffled. Fewer moves is more elegant.
It set off the world’s first puzzle craze in 1880s America. Puzzle king Sam Loyd offered $1,000 to anyone who could solve a board with 14 and 15 swapped — a position that is mathematically impossible. He never had to pay.
Lock the top rows first. Finish row one, then row two, then solve the bottom two rows column by column from the left. The final six tiles come together with a rotation. The worst case has been proven to need 80 moves.
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