Slide the tiles back into order, 1 through 15.

The famous $1000 puzzle nobody could solve

The 15 Puzzle dates back to 1874, invented by Noyes Chapman, but it became a genuine national craze a few years later when puzzle-maker Sam Loyd offered a real cash prize to anyone who could solve a specific starting layout with two tiles swapped. Nobody ever collected — that exact layout is mathematically impossible to solve, a fact that took mathematicians years to prove rigorously.

A solving strategy that scales to any size board: solve the top row first, then the left column, then treat whatever's left as a smaller version of the same puzzle. For the last two tiles in a row, there's a specific rotation trick — place them both in the row below, one column short of where they belong, then rotate them into place together.

Every board here is shuffled from the solved state using only legal moves, so unlike Loyd's famous trap, whatever you're handed is always solvable — no need to worry about parity, just about move count.