What this is
Turn off every light on a 5×5 board. Pressing a square flips it and its four orthogonal neighbours — on becomes off, off becomes on.
Your score is the number of presses. Fewer is better.
Controls
Tap or click a square. That is the whole game.
A corner flips 3 squares, an edge flips 4, and the middle flips 5. Anything that falls off the board is simply ignored.
Unsolvable boards never appear. Each board is built by pressing randomly from an all-off state, so retracing those presses always solves it.
Two rules worth knowing
Order does not matter
Pressing the same squares in a different order gives the same result. Only the set of squares matters. There is no “I should have done that first” in this game.
Pressing twice undoes itself
Press once and it flips; press again and it is back. So there is never a reason to press the same square twice — those two moves are pure waste. An optimal solution presses each square either once or not at all.
Together these mean the answer is a single question: which set of squares do you press?
Strategy — chase the lights
There is a standard method, usually called chasing.
Step 1: sweep top to bottom
Leave the top row alone and start on the second row. Whenever the square directly above is lit, press the one below it. Work down row by row and every row you pass turns off. When you reach the bottom, only the last row is still lit.
Step 2: read the bottom row, fix the top row
This is the trick. The pattern left on the bottom row tells you exactly which squares to press in the top row. The common cases:
| Lights left on bottom | Press on top row |
|---|---|
| 1st | 2nd |
| 2nd | 1st and 2nd |
| 3rd | 4th |
| 1st and 2nd | 3rd |
| 1st and 3rd | 1st |
Press those, then run step 1 again and the board clears.
Why it works
The top row decides everything else. From the second row down, “the only square that can turn off the one above” is forced, so there is no choice left. There are 32 possible top rows, exactly one of which works, and the table above is that lookup.
Using fewer moves
Chasing clears the board but does not always use the fewest moves.
If you are chasing a record, find the solution by chasing, then check whether any presses can be dropped. A 5×5 board has square sets that change nothing at all when pressed together, so removing the overlap gives the same result in fewer moves.
Learn to clear boards reliably first; optimise later.
Daily challenge
A new board opens every midnight (KST) and everyone solves the same one. Since order has no effect, the competition is purely about which squares you chose.