Nurikabe: pools and one sea
Anything with one way out must use it. And where three cells of a square are sea, the fourth is island.
How it works
Once the walls are up, two rules do most of the work. Both are about room to grow.
One way out: an island that still needs cells must grow through an open side. If it has just one, that cell is island. The sea must be one piece, so a piece of sea with one open side grows through it too.
Pools: no 2×2 square may be all sea. When three cells of a square are sea, the fourth is island. A square like that is a pool waiting to happen.
When the sea gets cut off
The one-sea rule also works backwards. If shading a cell would leave some sea with no way to reach the rest, that cell can’t be sea. Solvers check this by supposing and looking for a trapped piece.
Worth memorizing
One open side means grow there.
For an unfinished island, and for a piece of sea.
Three sea cells in a square: the fourth is island.
The sea is one piece.
Never wall a piece of sea in.
Practice
0 of 4 rightWhat is the ringed cell?
What is the ringed cell?
What is the ringed cell?
What is the ringed cell?