Most broken Light Up grids come from one of six mistakes, and the biggest is placing a bulb before it’s certain. The others are misreadings: light passing a black square, counting diagonal bulbs, treating a plain black square as a 0, forgetting a bulb lights its own square, and losing track of squares that were already ruled out. Each one is below, with how to spot it.
If you’re new, the rules and how to solve pages explain the ideas these mistakes go wrong on.
1. Placing a bulb before it’s certain
When a square might need a bulb, it’s tempting to try one and see. Often nothing tells you. We took every one of the game’s levels, put a wrong bulb on the first white square where the answer has none (one that broke no rule yet), and kept making honest moves until the game showed a red mark:
| Track | Red mark appeared | Moves before it (median) | Longest |
|---|---|---|---|
| Easy 7×7 | 39 of 50 | 4 | 14 |
| Medium 10×10 | 36 of 50 | 7 | 22 |
| Hard 12×12 | 32 of 50 | 16 | 49 |
On the rest, the red mark never appeared: the honest moves ran out first, with the grid wrong. Here’s the longest wait on Easy, level 49, with a wrong bulb in the top-left corner:
Fourteen moves later, a number at the bottom of the grid turns red. It’s nowhere near the wrong bulb.
How to avoid it: only place a bulb when a technique makes it certain. If you’re not sure, leave the square plain.
2. Letting light pass a black square
A black square stops light dead, in every direction. Two bulbs in the same row are fine if a black square stands between them, and a square behind a black square isn’t lit by a bulb on the other side.
How to avoid it: trace the light from each bulb outwards, and stop at the first black square.
3. Counting diagonal bulbs
A number counts the bulbs that touch its black square side by side: above, below, left and right. Bulbs on the diagonals don’t count, and neither do bulbs further along the row.
How to avoid it: check the four squares that share an edge with the number, and only those.
4. Treating a plain black square as a 0
A black square without a number has no rule of its own. It can have any number of bulbs beside it, from none to four. Only a 0 means none.
How to avoid it: use plain black squares only as walls that stop light.
5. Forgetting a bulb lights its own square
A square can be lit by a bulb of its own. A dark square boxed in by black squares and dots isn’t stuck: it needs a bulb right there.
How to avoid it: when you count the places that could light a square, count the square itself.
6. Losing track of what’s ruled out
Most of the work in Light Up is ruling squares out: on the game’s levels, 60 to 66% of all steps place a dot, not a bulb. Without the dots, it’s easy to miss that a dark square has only one way left to be lit.
How to avoid it: dot every square a bulb lights, every square beside a satisfied number, and every square ruled out some other way.
How to recover from a mistake
- A red bulb or number means a rule is broken, but the mistake may be elsewhere, often a long way back. Undo to the last move you were sure of.
- Press Stuck? It rings a wrong bulb, or a dot where a bulb belongs, before anything else.
- Stuck with no red mark? Check every number and its diagonals, then look at when you get stuck.