An island with all its squares is closed in: every square around it is wall. A 1 is complete from the start, which makes it the best first move.
This is one of the Nurikabe techniques. Each page shows its technique on the same 8×8 from the game, Medium level 8.
The rule
- Find an island whose squares (its number and the dots joined to it) add up to its number.
- Shade every square that touches it side by side.
In a real puzzle
On the 8×8 every technique page follows, it’s the very first move. The 1 in row 1, column 3 is complete, so the three squares beside it are wall.
How often you’ll need it
On every one of the game’s 150 levels: 5.1 times per Easy level on average, 9.3 on Medium and 14.8 on Hard. It’s 10 of the 42 steps on the 8×8 here.