How to Solve Light Up Puzzles

Controls on the page

Tap a white square once for a lamp, a second time for a dot, and a third time to clear it (an option swaps the order to dot first). A dot is your own note for "no lamp here" and is never checked. Long press or right click puts a dot straight away, and dragging across several squares paints dots along the way. Lit squares glow, so you can see at a glance which parts of the room are still dark. With Show mistakes on, lamps that shine on each other and numbers with too many lamps beside them turn red. On a computer the arrow keys move, Space or Enter changes a square, L places a lamp and D a dot.

Sizes and difficulty

Before a grid is published, a program solves it with exactly the techniques in this guide. If it gets stuck, the grid is changed until it doesn't, which also proves the answer is the only one.

Start with the 4s and the 0s

The quickest squares on any grid sit next to the extreme numbers.

1. A 4 has a lamp on every side. Place all four lamps at once. Each one lights its row and column, so a 4 often brightens a large part of the grid in one go.
2. A 0 has no lamp on any side. Dot its four neighbours. A 0 next to the edge or another block still means dots on the open sides.

Every square a new lamp lights can no longer hold a lamp, so the light and the dots together soon rule out many squares.

Threes, corners and full numbers

A number is easy whenever its open sides and its lamps match up. Count, for each number, the lamps it already has and the sides that could still take one (white squares that are not lit and not dotted).

• Exact fit. If the open sides are exactly as many as the lamps it still needs, they are all lamps. A 3 against the edge of the grid, a 3 next to another block and a 2 in a corner of the grid are the classic cases.
• Full. If the number already has all its lamps, every other side gets a dot.
• Too few. A 2 with one lamp and one open side left needs that side, whatever else is around it.

As lamps go in, recount the numbers nearby. One new lamp often turns a neighbouring 2 or 3 into an exact fit.

Dark squares only one lamp can reach

Every white square must be lit by some lamp: its own, or one in its row or column. For a dark square, list the squares that could still hold that lamp. Squares that are lit, dotted or behind a block don't count. When only one square is left on the list, that square is a lamp.

This happens most often in short dead ends between blocks, and in squares that you dotted earlier. A dotted square still needs light, so it is a good place to look.

Shared sight: squares every lamp would light

The most useful idea on medium and hard days is about pairs of squares that see each other. Suppose you know that at least one of two squares must hold a lamp, but not which one. Any third square that both of them would light can never hold a lamp: whichever of the two gets its lamp, it would shine on the third.

That gives two everyday patterns.

• Around a number. A 3 with four open sides must use three of them, so any two neighbouring sides hold at least one lamp. The square diagonal between those two sides sees both of them, so the four diagonal squares of such a 3 are all dots. A 1 in a corner of the grid works the same way: one of its two sides holds the lamp, so the square diagonal to the 1, which sees both sides, is a dot. A 2 with three open sides gets dots on the two diagonal squares away from its closed side.
• Around a dark square. If a dark square can be lit from only two squares, one in its row and one in its column, the square where their lines cross (the fourth corner of the rectangle they make) sees both of them, so it gets a dot, as long as no block stands in the way.

Shared sight never places a lamp by itself, but the dots it adds often leave a dark square with a single choice, and the grid starts moving again.

A routine for every grid

1. Fours and zeros, then every number whose open sides match the lamps it needs.
2. Dark squares with one choice. Sweep the grid for squares only one place can light.
3. Shared sight around threes, corners and dark squares with two choices.
4. Recount the numbers near every new lamp and go round again.

Common mistakes

• Counting diagonal lamps. A number only counts its four sides.
• Forgetting that light stops at blocks. Two lamps in the same row are fine if a block stands between them.
• Leaving a dotted square dark. A dot means "no lamp here", not "no light needed". Every white square, dotted or not, must be lit at the end.

Scoring and the leaderboard

Light Up is ranked by time, fastest first; fewer changes break ties. Dots are your own notes, but each tap counts as a change, so mark only the dots that help. The grid is checked the moment the last square is lit with every rule kept, and only a member's first solve of the day counts for the leaderboard. Practice puzzles from Play more are never ranked. If you enjoy placing things where they can and cannot go, try Tents & Trees and Rectangles next.