← back to Classic Puzzles

Lights Out

Tap a light to flip it and its neighbours, and switch them all off. 30 levels, hints, and the maths behind it.

Why Lights Out can always be solved

Every level is made by pressing random lights on an all-off board, so there is always a way back. Pressing a light twice is the same as not pressing it, and order does not matter, so each light needs at most one press. Mathematicians write the puzzle as a system of equations modulo 2. The "light chasing" trick: press the light below every lit one, row by row, down to the bottom.

tags: #puzzle #logic #math

Tap a light: it and its neighbours (up, down, left, right) switch on/off. Turn every light off! Keyboard: arrows to move, space/enter to press.

level 1/30moves: 0 · highest level: 0
Why can it always be solved?

Every level is made by pressing some random lights starting from a board that is all off. So there is always a way back: press the same lights again.

Pressing the same light twice is the same as not pressing it, and the order of presses does not matter. So each light needs to be pressed only 0 or 1 times.

Mathematicians write this puzzle as a system of equations with only 0s and 1s (algebra modulo 2). The "light chasing" trick: clear the top row by pressing the light right below every lit one, and keep going down to the bottom row.

How to install the app

This browser has not offered an automatic install button yet. Install it from the browser menu:

Android (Chrome)
Tap the ⋮ menu at the top right, then choose "Install app" or "Add to Home screen".
iPhone / iPad (Safari)
Tap the Share button (the square with an up arrow), then choose "Add to Home Screen". On iOS this only works in Safari.
Computer (Chrome / Edge)
Click the install icon (a monitor with a down arrow) at the right end of the address bar, or menu ⋮ → "Install Daily Apps".