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(1) Shade some of the cells. (2) The unshaded cells form islands of orthogonally connected cells. Every island contains exactly one number, and its size equals that number. (3) All shaded cells connect orthogonally into one wall. (4) No 2×2 block may be fully shaded. Numbered cells cannot be shaded.

Each tap on a cell cycles shaded → dot (·) → empty. A dot is a note meaning "this cell is part of an island" and does not affect the solution. Right-click toggles just the dot. On a keyboard, choose a cell with the arrow keys and press Space or Enter to cycle it.

The cells next to a 1 are always shaded. A cell squeezed between two numbers is shaded too, since leaving it white would join two islands. From there, repeat: the last cell of a 2×2 with three shaded cells is white, a wall with only one way out must extend through it, and a cell no number can reach is shaded.

Yes. Every puzzle has been checked by a program to have exactly one solution. The puzzles are generated in advance and served in date order.

By the time from your first tap to solving the puzzle, fastest first (the timer starts on your first move). You only need a nickname, and you can submit up to 30 times a day per size. The server checks that your board follows the rules.