PAPER / VARIANT
Fog Sudoku
The board starts dark. Every correct digit lights up its neighbours.
Click a cell, then a digit. Arrow keys move, 1–9 place, Backspace clears.
Rules
Fog hides every cell except the givens and the cells around them. Placing a correct digit clears the fog around it; a wrong digit clears nothing, so mistakes cost information rather than time.
- Hidden cells show only a question mark.
- A correct digit reveals itself and all eight neighbouring cells.
- Givens reveal their neighbourhood at the start.
- Ordinary sudoku rules apply underneath — the fog is only a view.
- A wrong digit reveals nothing at all.
Good to know
Ordinary sudoku rules still apply underneath the variant: every row, column and 3×3 box holds 1–9 exactly once. The extra rule sits on top and is usually what breaks the puzzle open.
More sudoku
All sudoku variants ↗Or read how to play Sudoku ↗.