Akari

The rules. Place light bulbs in white cells so every white cell is lit. Bulbs send light horizontally and vertically until a black cell or the edge of the board blocks it. No two bulbs may see each other. Numbered black cells must have exactly that many bulbs in their four orthogonal neighbors (cells touching the number directly).

Placing and removing. Click a white cell to cycle: empty → bulb → ×-mark → empty. The ×-mark records "I know there's no bulb here" — it doesn't affect the rules, just your bookkeeping. Right-click a white cell to toggle the ×-mark directly.

Mark mode. Press M (or the Mark mode button) to flip the primary tap to cycle the ×-mark first instead of the bulb. Touch-friendly alternative to right-click.

Light propagation. Cells lit by a bulb take a soft amber tint so you can see your coverage at a glance. Cells you haven't lit stay white.

Conflicts. If two bulbs can see each other along a row or column with no black cell between them, both cells turn rose-pink until you remove one. A numbered cell whose neighbor bulb-count doesn't match shows its number in rose.

Hint. Press the Hint button (or H) for a one-line explanation of the next forced placement or non-placement. The page never places the bulb for you.

Sizes. 7×7 plays in a minute or two; 20×20 is a real session. Every puzzle at every size is uniquely solvable by pure logical deduction — you'll never have to guess.

Keyboard shortcuts. M toggles Mark mode. H requests a hint. P pauses or resumes the timer. Backspace / Delete clears the most-recently-edited cell.