Completion Pool
Hard
Used by Smart Hint
A cell touching an island one short of its size would finish it — and a finished island is walled in sea on every side. If that walling would trap a 2×2 pool, the cell can't be land after all: it's sea.
- Sea
- Cells that establish the pattern
How it works
- A land cell touching an island always belongs to it — two different islands can never touch — so a cell beside an island one short of its size would be its final cell.
- Finishing an island seals it: every cell around it becomes sea. If those forced sea cells would complete a 2×2 pool with sea that's already on the board, the move breaks the rules before it's even finished.
- So the cell can't be land at all — it's sea. This pattern was the single most common reason a puzzle otherwise needed trial-and-error, which is why it has its own name here.