How to play Fireflies, the Light Up puzzle

The puzzle behind Fireflies: Light Up

Light Up is a logic puzzle from Nikoli, the Japanese puzzle publisher behind many classic pencil puzzles. In Japan it is called Bijutsukan, which means art gallery, a name that pictures lamps lighting up the rooms of a gallery. Many players outside Japan know it as Akari, the Japanese word for light.

Also known as Akari, Bijutsukan.

Fireflies keeps the classic rules. The lights are fireflies, the black walls are stones, and a stone can be plain or carry a number from 0 to 4. Meadows are at most 10 squares wide so they fit a phone, and the biggest are 10 wide and 12 tall. A firefly in the wrong place costs one of three hearts, unless you play in relaxed mode.

How to play Fireflies

Harry the hedgehog is scared of the dark. Place fireflies until his whole meadow is lit.

• A firefly lights its own square and shines along its row and column until a stone or the edge stops it.
• No two fireflies may shine on each other.
• A number on a stone says how many fireflies sit right beside it, above, below, left or right.
• Every meadow has exactly one answer. The hardest ones may need a what-if: if a square left dark gets the meadow stuck, a firefly goes there.
• A firefly in the wrong place costs a heart, and you have three. Relaxed mode has no hearts.
• Lose no hearts for three stars. A hint or two caps the meadow at two stars, and lots of hints at one.

What you can never do

• Two fireflies in one line, unless a stone stands between them.
• More or fewer fireflies beside a number than it says.

Controls with Harry the hedgehog

• Tap a square to put a dot on it, a note that no firefly goes there. Tap again to clear it.
• Double tap or press and hold to place a firefly. With the Firefly tool on, a single tap places one.
• Drag to dot several squares at once.
• With a keyboard, use the arrow keys to move, Space for a dot, Enter for a firefly and Backspace to clear.
• Undo takes back your last move.

Strategies for beginners

Start with the 4s, the 0s and the tight spots

A 4 needs a firefly on every side. A 3 against the edge or another stone has only three free sides, so all of them get one. A 0 never has a firefly beside it, so dot those squares straight away.

Dot what is already done

Once a number has all its fireflies, no other square beside it can hold one: dot them. Squares already lit by a firefly can never hold another one, because the two would shine on each other.

Fill a number with no spare sides

Count the free squares beside a number. When there are exactly as many as it still needs, each of them gets a firefly. A 2 tucked in a corner of stones is a common example.

Light the lonely squares

Look for a dark square that only one free square can still light, often in a pocket between stones. A firefly has to go on that one square, or the dark square would stay dark for good.

Keep away from the corners of a 3

A firefly diagonal to a 3 would light two of its sides at once, leaving it at most two fireflies. So squares touching a 3 at a corner never hold a firefly. The same thinking helps around 2s with few free sides.

Ask what a firefly would block

Before placing a firefly, check which squares it would light. If that leaves a number without enough free sides, or leaves a dark square with no way to be lit, the firefly cannot go there.

Try a what-if on the biggest meadows

Master meadows need one, and so do some Expert ones. Imagine a square left dark and follow the logic. If the meadow gets stuck, with a number short or a square that can never be lit, a firefly goes on that square.

Quick tips from the game

• Start with the numbers: a 4, or a 3 against the edge, fills every free side.
• Dot the squares beside a 0 and beside a stone that already has all its fireflies.
• Look for a dark square that only one free square can still light.

Words to know

Light

The piece you place, also called a bulb or lamp. In Fireflies every light is a firefly, and it lights its own square too.

Wall

A black square that stops light, also called a block. In Fireflies the walls are stones, some of them numbered.

Number

A count on a wall of how many lights touch it on its four sides. Lights at its corners do not count.

Lit square

A square a light shines on. A finished puzzle has every open square lit, and no light shining on another.

Dot

A mark for a square that cannot hold a light. Dots are your own notes and are never checked.

What-if

Trying one choice and following the logic until it works or breaks. Only the hardest meadows need it.

Questions about Fireflies