Dusk

Puzzle · plays in your browser, no download

Press a light and it flips, with its four neighbours. Turn them all off — the fewest presses it takes is shown.

Where it comes from

Lights Out, the handheld with five by five buttons from the mid-nineties. Underneath it is linear algebra: because order never matters and two presses cancel out, a solution is a set of squares and can be calculated. That is how the game knows the fewest presses — and how it knows which boards to never deal you.

How to play

The number on the right is how few presses it can be done in — it is calculated, not guessed, so it is always reachable.
That is why every board here can be solved: it is built by pressing, not by lighting squares at random.
Three stars: six by six, and around sixteen presses at best.

What makes this one different

The number beside the board is the fewest presses it can be done in, and it is calculated rather than guessed, so it is always reachable. Order never matters and pressing the same light twice does nothing, which means a solution is just a set of squares. On a four by four nearly half of all random patterns can never be turned off — so none of those are ever dealt to you.

▶ Play Dusk