How to play Light Switch: the chase-the-lights method

Light Switch looks like it needs trial and error, but there’s a reliable method that solves every board. It’s called chasing the lights.

The rules

Some lights are on. Tapping a light switches it and the lights directly above, below, left and right of it. Diagonals don’t change. Turn every light off to win.

Two facts that make it easier

• Order doesn’t matter. Tapping A then B gives exactly the same board as tapping B then A.
• Two taps cancel out. Tapping the same light twice changes nothing, so a solution never needs any light tapped more than once.

Together, these mean a solution is just a set of lights to tap once each.

The method: chase the lights down

1. Look at the top row. For every light that’s on, tap the light directly below it. That turns the top one off without touching the rest of the top row.
2. Now do the same for the second row: for each light still on, tap the one below it.
3. Keep going down. When you reach the bottom, every row is dark except possibly the last one.

Fixing the last row

If the bottom row is dark, you’re done. If not, the pattern left in the bottom row tells you which lights to tap in the top row. Tap those, then chase the lights down again. This time the board clears.

On a 5×5 board, these are the patterns to remember. Read the bottom row from left to right, where X is a light that’s on:

• Bottom row X X X . .: tap the 2nd light in the top row.
• X X . X X: tap the 3rd.
• X . X X .: tap the 5th.
• . X X . X: tap the 1st.
• . X . X .: tap the 1st and 4th.
• . . X X X: tap the 4th.
• X . . . X: tap the 1st and 2nd.

Then chase the lights down once more and the board goes dark. These seven patterns are the only ones that can be left on a 5×5 board.

Other board sizes have their own tables. If you’d rather not learn them, the Hint button always shows a light that’s part of a solution.

Going for fewest moves

Chasing always works, but it isn’t always shortest. The Fewest number on the board is the shortest solution. To match it, look for groups of lights that a single tap would switch off together.