Norinori

A deceptively simple shading puzzle from Nikoli, named after the strips of seaweed it leaves scattered on the board. The grid is split into bold-outlined regions and you shade exactly two cells in each. The catch is that every shaded cell must pair off with exactly one neighbour, so the shading always ends up as separate two-cell dominoes — and those dominoes are free to cross region borders. Each 7×7 board has exactly one solution.

How to play

The board is carved into regions by the thick outlines. Your job is to shade exactly two cells inside every single region — no more, no less.

The second rule is what makes it a puzzle. Every cell you shade must be orthogonally adjacent to exactly one other shaded cell. Not zero, not two. So the shaded cells always come out as separate dominoes: pairs of two touching cells, with no pair ever touching another pair.

Crucially, a domino does not have to stay inside one region. It may lie half in one region and half in the next. That is why you cannot solve a region on its own — the two cells a region owes are often paired with partners on the far side of its border.

Tap a cell to shade it. Tap it again to leave a small ✕, which marks a cell you have worked out must stay empty — it costs nothing and keeps your deductions straight. A third tap clears it. A region's outline turns green the moment it holds exactly two shaded cells and red if you overshoot, and a shaded cell turns red if it ends up touching two others. Every board has exactly one solution, and your finishing time is your score.

Tips & strategy

Hunt for the small regions first. A region made of exactly two cells is a gift — both of its cells must be shaded, no thinking required, and that pair immediately forbids shading in every cell that touches it. Regions of three or four cells are nearly as generous, because the two cells you owe have very few places to sit.

Use the ✕ marks aggressively, because "definitely empty" is where the deductions actually come from. The moment a region has its two shaded cells, every other cell in that region is empty — mark them all. Those fresh ✕ marks usually strand a shaded cell somewhere nearby with only one possible partner left, which forces the next domino for free.

Remember that a shaded cell needs a partner, not just a home. If a cell's every orthogonal neighbour is already ruled out, that cell can never be shaded, no matter how much room its region has. Scanning for cells that have been quietly starved of neighbours is the fastest way to break a stuck board.

Watch the borders, not the regions. When a region's two shaded cells are forced apart — too far to touch each other — you know both of them must pair outside, which pins down cells in the neighbouring regions before you have even looked at those regions properly. Conversely, a domino placed entirely inside one region finishes that region on the spot, so ask whether a region can even fit a whole domino before assuming it does.