Islands and one sea
Nurikabe is a Japanese shading puzzle. You decide which squares are wall, drawn here as blue sea, and which are island, drawn as sand. The numbers fix the size of each island. Three other rules shape everything else: the sea must be one connected body, it can never form a 2×2 pool, and two islands may never touch side-to-side.
You are mostly placing sea, not islands. Every unshaded square counts as island in the end, and a dot only helps you remember a square you have proved is island.
First moves on any board
A few deductions are available before you think hard about anything.
- Ones are already finished. A 1 is an island of one square, so all four squares touching it are sea.
- Squares between two numbers. A square that touches two different numbered squares, or sits between two islands you have started, must be sea. If it were island, it would merge them.
- Squares no island can reach. Count outward from each number. A square that is farther from every number than that number's size allows is out of range for all of them, so it must be sea.
Here is a worked example on the 5×5 Easy board. Say a 2 sits in a corner. It has only two squares touching it, and the island needs exactly one more square, so one of the two is island and the other is sea. You cannot choose yet. But look at the square diagonally in from the corner. It touches both candidates, so whichever one becomes island, that diagonal square sits next to a full island of 2. It must be sea either way. Shade it.
Keeping the sea joined and pool-free
Once the obvious walls are down, the sea rules take over.
- No 2×2 pools. Whenever three squares of a 2×2 block are sea, the fourth must be island. That island square then has to belong to some number, which often tells you which island grows toward it.
- A sea with one way out. If a stretch of sea is boxed in and has just one empty square leading out, that square must be sea too. Otherwise that stretch would be cut off from the rest.
- Close finished islands. As soon as an island reaches its number, it turns green, and every square touching it must be sea. Shade them straight away so the sea can keep connecting.
- Islands with one direction to grow. If an unfinished island has only one empty square to expand into, that square is island. Mark it with a dot and keep counting.
What usually goes wrong
- Forgetting the dots are optional. Players sometimes try to dot every island square and lose time. Dot only squares you have proved are island and want to keep in view.
- Building a pool without noticing. On 7×7 and 9×9 boards, a 2×2 block of sea is easy to miss in a crowded corner. Scan for pools each time you shade a cluster.
- Stranding a piece of sea. It is tempting to finish an island along an edge and accidentally wall off a pocket of sea behind it. Before you close an island, check that the sea around it still has a way out.
- Letting two islands drift together. When two unfinished islands grow toward each other, the square between them must be sea. Place it early.
Learning curve across the sizes
Easy is a 5×5 grid with three or four small islands. It is a good place to practise the opening deductions and the corner trick above until you can spot them at a glance.
Medium is 7×7 with five to seven islands of up to five squares. Here the no-pool rule and the one-way-out rule start to decide whole regions. Hard is 9×9 with seven to fourteen islands of up to six squares, and the connection rule becomes the main tool: you will often find the last few walls by asking where the sea still needs to pass.
The hint first removes a wall that does not belong, then shades a square that should be sea. Each board has one solution, so a hint never leads you astray, but try to name the rule behind it before moving on. Hitori at /games/hitori uses a similar "keep everything in one piece" rule for its unshaded squares.
