Light Up asks you to place lights until every open cell is lit, without any two lights shining on each other.
How light travels
A light brightens its own cell and shines in straight lines along its row and column until it hits a wall. It doesn’t bend round corners or pass through walls. If two lights can see each other along a row or column with no wall between them, something’s wrong. Numbered walls add one more rule: the number says exactly how many lights sit in the cells directly above, below, left and right of the wall. Diagonals don’t count.
Numbered walls first
A 0 means none of its neighbours can hold a light. A 4 means all of them must. In between, compare the number with the open cells around the wall. If a 3 has exactly three free neighbours, all three get lights. Once a wall has all the lights it needs, its other neighbours are off-limits.
After placing a light, take note of everything it lights up. Those cells are covered, and none of them can hold another light, because it would see the first one.
Worked example
One light satisfies both wall counts
- Each wall marked 1 needs exactly one light beside it. The corner walls and the edges of the board leave the centre as the only open neighbour of both.
- A light in the centre satisfies both numbered walls at once, and lights the whole middle row.
A teaching board, not one of today’s puzzles.
Where can the light come from?
Pick a dark cell and look along its row and column as far as the nearest walls. Which of those cells could still hold a light? If only one could, it must. This is how you make progress in parts of the board with no numbered walls nearby.
Counts, coverage and conflicts
Every numbered wall can be satisfied while a corner stays dark, and every cell can be lit by two lights that see each other. A finished board needs the right counts, full coverage and no conflicts, all at once. A hint can point to a light to place or one to rethink.
See all three at work in the complete Light Up walkthrough.