Nurikabe was named by the Japanese puzzle maker Nikoli. A nurikabe is an invisible wall from Japanese folk tales. The puzzle is also called Islands in the Stream or Cell Structure.
The rules
- Each island holds 1 number and has exactly that many squares.
- Islands only touch at corners. They never share a side.
- Every other square is sea.
- All the sea must join up along the sides of its squares.
- No 2 by 2 block may be all sea.
Quirk uses the classic rules. Printed puzzles often shade the sea black and leave the islands white. Quirk calls them sea and island instead. Boards go up to 8 by 8.
A worked example
Here is a small board, 3 squares wide. There is a 1 in the top left corner and a 4 in the top right corner. All other squares start open.
- The 1 is already a full island. So the squares beside it are sea: row 1, column 2 and row 2, column 1.
- The sea must join up. So the sea square between the 1 and the 4 must reach the other sea square.
- Its only open neighbour is the middle square. So the middle square is sea too.
- The 4 needs 3 more squares. Its only way out is down, to row 2, column 3. Make that island.
- The square to its left is the middle, which is sea. So the island must grow down again, to row 3, column 3.
- The 4 still needs 1 more square. The only open square next to it is row 3, column 2. Make that island.
- The last open square is row 3, column 1. It would make the 4 too big, so it is sea.
- Check the sea. Its 4 squares all join up along their sides.
- Check the bottom left 2 by 2 block. It has one island square, so it is not all sea.
The finished board looks like this.
| Row | Column 1 | Column 2 | Column 3 |
|---|---|---|---|
| 1 | 1 | sea | 4 |
| 2 | sea | sea | island |
| 3 | sea | island | island |
Beginner techniques
Close off full islands
When an island has all its squares, every square beside it is sea. A 1 is full from the start, so mark sea around every 1 first.
A square between two numbers
Some squares touch 2 islands that each have a number. Making such a square island would join them. So it must be sea.
Squares no island can reach
Count how far each island can still grow. A 2 can only reach the squares right next to it. A square that no island can reach must be sea.
The only way out
If an island still needs squares and has only 1 open square beside it, it must grow there.
The sea works the same way. If a part of the sea has only 1 way to join the rest, that square is sea.
No 2 by 2 of sea
Look at each 2 by 2 block. If 3 of its squares are sea, the fourth must be island.
Common mistakes
- Letting 2 islands touch along a side, so they become one island with 2 numbers.
- Leaving a 2 by 2 block that is all sea.
- Cutting the sea into 2 parts that no longer join.
- Making a closed-off island with no number in it.
- Making an island bigger or smaller than its number.
Questions
Is Nurikabe the same as Islands in the Stream?
Yes. Islands in the Stream and Cell Structure are other names for the same puzzle. The rules are the same.
Do I have to mark every island square?
No. You can leave island squares open. They count as island when the board is checked.
Can islands touch at the corners?
Yes. Islands may touch at a corner. They may never share a side.
Does every puzzle have one answer?
Yes. Every Quirk board has exactly one answer. You can reach it by thinking it through, without guessing.
What sizes are there?
Quirk starts with small boards and small islands. The biggest boards are 8 by 8.
Is it good for beginners?
Yes. Short lessons show each idea, one at a time. If you get stuck, a hint points to the next step. You can also undo a move or reset the board.
Related puzzles
Playing it in Quirk
Tap a square to make it sea. Tap again for island, and once more to clear it.
Drag over squares to give them all the mark the first one gets. If an island grows too big or the sea splits apart, Quirk shows you where.
