Rules & objective
Turn every light off.
- A click toggles the selected light and its orthogonal neighbors: above, below, left and right. Diagonal lights are unchanged.
- At an edge or corner, only neighbors inside the board are affected. There is no wraparound.
- Every board is generated by applying toggles to an all-dark board, so a solution exists. Undo is available, and fewer moves is the better result.
Key terms
- Toggle
- Changing a light from on to off or from off to on.
- Cross
- The selected square together with its four possible neighbors.
- Parity
- Whether a square has been toggled an odd or even number of times.
Difficulty levels
| Level | What changes |
|---|---|
| Easy | 3 × 3 lights, mixed with four toggles. Every board is solvable. |
| Medium | 4 × 4 lights, mixed with seven toggles. |
| Hard | 5 × 5 lights, mixed with twelve toggles. Think in patterns. |
Changing difficulty starts a fresh round. Personal bests are tracked separately.
This version on puzzle free online
The boards are 3 × 3, 4 × 4 and 5 × 5. Their generation uses 4, 7 and 12 toggles respectively, but those counts do not guarantee a shortest solution because repeated toggles can cancel.
Common questions
May I click a light that is already off?
Yes. The effect on neighboring lights can make that the useful move.
Does the order of the same clicks matter?
The toggle effects commute: the same set of clicks gives the same final pattern, and clicking one square twice cancels its effect.