Most broken Nurikabe grids come from one of six mistakes, and the biggest is shading a square before it’s certain. The others are slips with the rules: a wall in pieces, a 2×2 block of wall, two islands joined, an island left without a number, and islands joined only at a corner. Each one is below, with how to spot it.
If you’re new, the rules and how to solve pages explain the ideas these mistakes go wrong on.
1. Shading a square before it’s certain
When a square might be wall, it’s tempting to shade it and see. Often nothing tells you. We took every one of the game’s levels, shaded the first square that’s island in the answer (one that broke no rule yet), and kept making honest moves until the game showed a red mark:
| Track | Red mark appeared | Moves before it (median) | Longest |
|---|---|---|---|
| Easy 6×6 | 48 of 50 | 8 | 26 |
| Medium 8×8 | 34 of 50 | 16 | 35 |
| Hard 10×10 | 36 of 50 | 13 | 41 |
On the rest, the red mark never appeared: the honest moves ran out first, with the grid wrong. Here’s the longest wait on Easy, level 40, with one wrong wall square in the top row:
Twenty-six moves later, a 2×2 block of wall appears at the bottom of the grid, nowhere near the mistake.
How to avoid it: only shade a square when a technique makes it certain. If you’re not sure, leave it plain.
2. Letting the wall fall into pieces
All the wall must join up side by side. Two pieces that only touch at a corner aren’t joined, and a piece walled in by islands can never join the rest.
How to avoid it: check every piece of wall still has a way out to the others. A piece with only one way out must go that way.
3. Forgetting the 2×2 rule
Four wall squares in a 2×2 block break the rules, however natural they look. Thick patches of wall are where it happens.
How to avoid it: whenever three walls meet in a 2×2 block, dot the fourth square.
4. Joining two islands
A square touching two different islands can’t be island: it would join them into one island with two numbers.
How to avoid it: shade every square between two islands as soon as you see it.
5. Leaving an island without a number
Every unshaded square must belong to an island with a number. A pocket of unshaded squares that no number can reach is a mistake waiting to happen.
How to avoid it: check how far each island can grow. A square no island can reach is wall.
6. Joining islands at a corner
Islands, like the wall, join side by side only. Two island squares that touch at a corner are in different islands, unless they’re joined some other way.
How to avoid it: count an island’s squares by following its edges, not its corners.
How to recover from a mistake
- A red outline or number means a rule is broken, but the mistake may be elsewhere, often a long way back. Undo to the last move you were sure of.
- Press Stuck? It rings a wrong square before anything else.
- Stuck with no red mark? Check what each island can reach, then look at when you get stuck.