Solving guide

How to solve Light Up (Akari) puzzles

Light Up, also called Akari, is a logic puzzle first published by Nikoli in Japan. In Firefly Fields the light bulbs are fireflies and the black walls are garden stones.

The three rules

Light every tile. A firefly lights its whole row and column until a stone or the edge stops the light.
Out of sight. Two fireflies may never see each other: on the same row or column, a stone must stand between them.
Read the stones. A number on a stone is exactly how many fireflies touch it: above, below, left or right. Diagonals don't count. Stones without a number can touch any amount.

A good puzzle has one solution and never needs a guess. You will place fireflies where they are forced, and put a dot on tiles that can't hold one, so you never look there again.

Seven techniques, easiest first

On each example board the soft teal outline shows the stones and tiles the trick looks at. Press Show the step to see what it tells you: a gold firefly to place, or gold rings for tiles that get a dot.

1. Fill the stone

A stone needs exactly as many fireflies as it has free neighbours left. Then there is no choice: every free neighbour gets a firefly. Look for a 2 in a corner, a 3 against the edge, or any 4.

2. Full stone

When a stone already touches as many fireflies as its number, none of its other neighbours can have one. Mark them with dots. A 0 stone is full from the start, so dot all its neighbours first thing.

3. Last light

Every dark tile has to be lit by someone. Check which tiles could still light it: if only one is left (sometimes the dark tile itself), that tile must hold a firefly. Dotted tiles are often what leaves a tile with a single option.

4. The diagonal

A stone that needs one fewer firefly than its free neighbours must use at least one tile of every pair of neighbours. A tile diagonal to the stone sees two of them, so a firefly there would block both: it gets a dot. A 3 with four free sides, or a 1 with two, are the classic cases.

5. Don't leave it dark

Before placing a firefly, ask what it would block. If it would leave some dark tile with no tile left that could light it, that spot can't hold a firefly: it gets a dot.

6. Shared need

If every way to light a dark tile goes through neighbours of the same stone, and that stone needs just one more firefly, that firefly is spent there. The stone's other neighbours get dots.

7. What if?

Imagine a firefly on the glowing tile and follow only the easy steps. If they end in a broken garden (a stone over its number, or a tile no one can light), the opposite must be true. It's one careful thought, never a guess. You only need it on the hardest nights.

A solving routine

Dot every neighbour of each 0 stone.
Fill the stones that have exactly as many free sides as their number (4s, corner 2s, edge 3s).
After each firefly, dot the full stones' other neighbours and look for tiles with only one way left to be lit.
Use the diagonal trick on 3s and on 1s with two free sides.
When nothing is forced, look for a tile whose firefly would leave another tile dark, then try "what if?".

Ready to try? A new 7×7 or 8×8 garden opens every day. Play today's Light Up puzzle, or get the app for 300 puzzles, a Technique Notebook with six practice boards per trick, and hints that point to the next step.

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Questions

Is Light Up the same as Akari?

Yes. Nikoli named the puzzle Akari ("light"); it is published in English as Light Up. Firefly Fields uses the same rules with fireflies instead of light bulbs.

Do I ever have to guess?

No. Every Firefly Fields puzzle is checked by a solver that only uses logical steps, and it has exactly one solution.

Does a stone without a number limit fireflies?

No. It only blocks light. Any number of fireflies may touch it.

Can a firefly light another firefly?

No, that is the "out of sight" rule: if two fireflies share a row or column with no stone between them, the garden is broken.