Light Up asks two questions at once: does each numbered wall have the right number of lights, and can every open cell be lit? This small board shows how the two questions help answer each other.
A quick reminder of the rules: a light shines along its row and column until it hits a wall, and lights its own cell too. Two lights can’t shine on each other. A numbered wall needs exactly that many lights directly above, below, left or right of it. Diagonals don’t count. The Light Up guide has more.
Row 1, column 1: plain wall | Row 1, column 2: unlit space | Row 1, column 3: unlit space | Row 1, column 4: unlit space |
Row 2, column 1: 1 wall | Row 2, column 2: unlit space | Row 2, column 3: unlit space | Row 2, column 4: unlit space |
Row 3, column 1: 0 wall | Row 3, column 2: 2 wall | Row 3, column 3: unlit space | Row 3, column 4: unlit space |
Row 4, column 1: unlit space | Row 4, column 2: unlit space | Row 4, column 3: 2 wall | Row 4, column 4: unlit space |
Starting board
Four numbered walls, eleven open cells
Rows run top to bottom and columns left to right. Dark squares are walls. The sun symbol is a light, and shaded cells are lit. In these diagrams an × marks a cell that can’t hold a light; you won’t need to mark these in the game.
1. Start with the tightest walls
The 1 in row 2, column 1 has only one open neighbour: the cell to its right. The edge of the board, the plain wall above and the 0 below block everything else, so that cell gets a light. It lights the rest of row 2 and the cell above it, and stops at the 2 below.
The 0 in row 3, column 1 says its only open neighbour, row 4, column 1, can’t hold a light. That cell still has to be lit by something, though. “No light here” doesn’t mean “this cell can stay dark”.
Row 1, column 1: plain wall | Row 1, column 2: lit space | Row 1, column 3: unlit space | Row 1, column 4: unlit space |
Row 2, column 1: 1 wall | Row 2, column 2: light | Row 2, column 3: lit space | Row 2, column 4: lit space |
Row 3, column 1: 0 wall | Row 3, column 2: 2 wall | Row 3, column 3: unlit space | Row 3, column 4: unlit space |
Row 4, column 1: unlit space, cannot contain a light | Row 4, column 2: unlit space | Row 4, column 3: 2 wall | Row 4, column 4: unlit space |
One forced light, one ruled-out cell
The outlined light satisfies the 1. The bottom-left cell can’t hold a light, so another light has to reach it.
2. Light the bottom-left corner
Light can’t reach row 4, column 1 from above, because the 0 is a wall, and the cell itself is ruled out. Looking right along row 4, the only open cell before the next wall is row 4, column 2, so the light must go there. This time the deduction came from a cell that needs light, not from a wall asking for one.
Row 1, column 1: plain wall | Row 1, column 2: lit space | Row 1, column 3: unlit space | Row 1, column 4: unlit space |
Row 2, column 1: 1 wall | Row 2, column 2: light | Row 2, column 3: lit space | Row 2, column 4: lit space |
Row 3, column 1: 0 wall | Row 3, column 2: 2 wall | Row 3, column 3: unlit space, cannot contain a light | Row 3, column 4: unlit space |
Row 4, column 1: lit space, cannot contain a light | Row 4, column 2: light | Row 4, column 3: 2 wall | Row 4, column 4: unlit space |
Only one cell can reach the corner
The 2 in row 3, column 2 now has lights above and below it. That’s its full count, so its last open neighbour, row 3, column 3, can’t hold a light.
3. Finish the bottom wall
The 2 in row 4, column 3 already has a light on its left. The cell above it was just ruled out, and below it is the edge of the board. The only place left for its second light is on its right, in row 4, column 4. That light also brightens the right-hand column.
Row 1, column 1: plain wall | Row 1, column 2: lit space | Row 1, column 3: unlit space | Row 1, column 4: lit space |
Row 2, column 1: 1 wall | Row 2, column 2: light | Row 2, column 3: lit space | Row 2, column 4: lit space |
Row 3, column 1: 0 wall | Row 3, column 2: 2 wall | Row 3, column 3: unlit space, cannot contain a light | Row 3, column 4: lit space |
Row 4, column 1: lit space, cannot contain a light | Row 4, column 2: light | Row 4, column 3: 2 wall | Row 4, column 4: light |
Every numbered wall is satisfied
Both 2s have their two lights. But row 1, column 3 is still dark, which is a good reminder that correct counts don’t guarantee a finished board.
4. One dark cell left
Which cell could light row 1, column 3? Its neighbours on either side are already lit by other lights, so a light in either would see one of them. Below it, row 2, column 3 is in the first light’s line of sight, and row 3, column 3 was ruled out by the satisfied 2. The wall in row 4 blocks the column after that. That leaves only the dark cell itself.
Row 1, column 1: plain wall | Row 1, column 2: lit space | Row 1, column 3: light | Row 1, column 4: lit space |
Row 2, column 1: 1 wall | Row 2, column 2: light | Row 2, column 3: lit space | Row 2, column 4: lit space |
Row 3, column 1: 0 wall | Row 3, column 2: 2 wall | Row 3, column 3: lit space, cannot contain a light | Row 3, column 4: lit space |
Row 4, column 1: lit space, cannot contain a light | Row 4, column 2: light | Row 4, column 3: 2 wall | Row 4, column 4: light |
Coverage finishes the job
The last light brightens the top row and the cells below it. All eleven open cells are lit, every numbered wall has its count, and no two lights can see each other.
Three checks for every board
After each light, check the numbered walls nearby, the cells it can see, and the cells that are still dark. A fully lit board can hide two lights that see each other, and correct counts can still leave a dark corner. You need all three.
This board was made for the lesson and checked for a single solution; it isn’t from the Daily set or the Weekly collection. Try Light Up in the Library when you’re ready for a fresh layout.