Lights Out
How to play
Click a tile to flip it and its 4 cardinal neighbors. The puzzle is solved when every tile is off. Easier than it looks.
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About Lights Out
Lights Out is a logic puzzle on a five-by-five grid of cells. Each cell is either lit or unlit. Click any cell and that cell flips state, plus its four cardinal neighbors flip too. The puzzle is solved when every cell on the grid is unlit. Sounds simple. The interaction between cells is what makes it interesting.
The format was published as a handheld electronic toy by Tiger Electronics in 1995. The original device had a five-by-five grid of buttons and an LED behind each one. Pressing a button toggled it and its neighbors. Players spent commute-length amounts of time trying to clear randomized starting patterns. The toy was a steady seller for over a decade.
The mathematics behind it is more interesting than the toy suggests. Lights Out is a linear system over the field with two elements. Every starting position has a deterministic solution if one exists. Some starting positions have no solution at all. Pixel Arena's version generates only solvable puzzles by applying a random sequence of clicks to a blank board, which guarantees the result can be reversed.
Controls are click or tap any cell. There is no other input. The state changes are immediate and visible. There is no undo. There is no hint system. The puzzle is solved when every cell is dark.
Three difficulty levels control how many random clicks are applied to the starting board. Easy uses three clicks, which produces a board with at most fifteen lit cells in a recognizable pattern. Medium uses six. Hard uses ten and produces patterns that are hard to read by inspection alone. The optimal solve count is always the same as the starting click count, but only if you find the exact sequence.
The starter strategy is the chase-the-lights method. Work top to bottom, one row at a time. For each lit cell in the current row, click the cell directly below it. This turns off the lit cell at the cost of changing the cells around it in the row below. Continue down the board. By the time you reach the last row, the only lit cells are in the last row.
The last row is the hard part. You cannot turn off cells with cells beneath them, because there are none. The fix is a known set of patterns. There are seven distinct unsolvable last-row configurations, and for each one there is a specific top-row click pattern that propagates down to clear the bottom. Memorize the seven patterns and the top-row counter-clicks for each, and you can solve any board in under a minute regardless of starting state.
Most casual players do not learn the formal solve method. They click intuitively until the board clears or they get stuck. Intuitive play often works on easy and sometimes on medium. Hard usually requires the chase-and-bottom-row pattern.
A good practice is to pay attention to which cells you have clicked. The order does not matter, only the parity. Clicking the same cell twice cancels itself out. Optimal solutions never click any cell more than once. If you find yourself clicking and unclicking, you are wasting moves. Step back, look at the board, and try a different region.
The version here saves your best move count per difficulty. The five-by-five grid uses bright yellow when lit and dark gray when unlit. The contrast is high enough to be readable in any lighting. There is no animation between states because animations would slow down the rapid trial-and-error that is part of the play loop.
Each puzzle is solvable. There is no situation where you can get stuck and have to reset. If you get to a configuration that looks impossible, it is not. Some sequence of clicks will clear it. The challenge is finding that sequence in fewer moves than your previous best.