How to play Lights Out
- Look at the board. Lights that are on glow warm yellow; lights that are off sit as dark wood. The default board is 5×5; you can switch to 3, 5 or 7.
- Tap a light. Each tap flips that light and its four neighbours (up, down, left, right) — a plus-shaped cross. Corners have fewer neighbours.
- Turn every light off. The puzzle is solved when the whole board is dark. Your press count is shown above the board.
- Use Undo, New or Hint. Undo reverses the last press (toggles are their own inverse). New deals a fresh solvable board. Hint presses one cell from a linear-algebra solution over GF(2).
Lights Out strategy
- Chase lights down a column: clear a lit cell by pressing the cell directly below it, then finish the last row with the linear solution.
- Corners only flip three cells — they are powerful openers when a corner is lit.
- Order does not matter and pressing twice cancels, so you never need more than one press per cell in an optimal plan.
- If you are lost, Hint plays one press from a full GF(2) solution; games that used Hint do not update your best.
Why the puzzle is solvable (linear algebra over GF(2))
Each press is addition mod 2 on a set of cells. Stacking the 25 possible presses for a 5×5 board gives a 25×25 matrix over GF(2). Solving Ax = b with Gaussian elimination finds which cells to press. Our sample seed-20261023 board needed 14 presses in one particular solution, and replaying those presses leaves every light off.
That is also how Hint works — the same solver the unit tests replay on random reverse-generated boards.
More logic puzzles
Like planning ahead? Try peg solitaire, Tower of Hanoi, the 15 puzzle, or grid logic like Fillomino and LITS. Chinese speakers can play the 关灯游戏 version of this page with the same saved games.