An example Fog of War puzzle: only a few cells are lit; the fog lifts around each correct digit you place.
Fog of War Sudoku hides the grid under a layer of "fog". Only a few starting cells are lit; the rest are dark. As you place correct digits, the fog lifts around them, revealing more of the puzzle - so the grid uncovers itself as you solve.
What the fog hides
This is the part that decides how the variant plays: the fog hides whatever lies beneath it. Not just empty cells, but also given digits and the puzzle's own markings - Killer cages and their sums, thermometers, whisper lines. A clue you have not uncovered is a clue you do not know exists.
That is why Fog of War is always played on top of another variant, never on plain Classic Sudoku. In a Classic grid the only information is the digits you can see; cover them and there is nothing left to reason from. A variant's markings are local - a cage summing to 3 across two cells pins them to 1 and 2 without you seeing anything else on the board - so they keep the solve moving as they come into view. Puzzles pairing fog with Killer, thermometers or line constraints are the norm; a fog puzzle over bare Classic is not something setters produce.
The rule: correct digits clear the fog
Every time you place a digit that is right, the fog clears from that cell and the eight cells around it, exposing whatever they hold. A wrong digit reveals nothing - which is feedback in itself, and the reason you never need to wonder whether you have gone astray. You may also write into a fogged cell; you do not have to see it first. Below, a lit cell holds a 5 while the fog still covers its neighbours.
A single lit cell (5) surrounded by fog (dark cells).
A worked example, step by step
Start with almost everything fogged and one visible clue.
Step 0. One clue (5) is lit; everything else is under fog.
Step 1 - place a correct digit. Using the visible clues and the ordinary rules, you work out that the cell to the right must be a 2 and place it. Because it is correct, the fog clears around it.
Step 1. The correct 2 clears the fog around it, revealing new cells (light).
Step 2 - use what's revealed. The newly-lit cells may contain fresh clues, which in turn let you place more digits and clear yet more fog. The solve becomes a chain reaction: each correct placement opens up the next part of the puzzle, and the whole grid gradually comes into view.
How to play it well
The golden rule is never guess. A wrong digit doesn't clear anything and just clutters the board; worse, on some versions it can be hard to tell a guess from a deduction later. Solve from certainties only, and treat the fog as confirmation that you are on the right track. The grid underneath has a single solution and no guessing is needed - but remember that your certainties can only rest on clues you have actually uncovered, so the reasoning always starts at the edge of the lit area.
Solving strategy
- Only place certainties. Fog rewards correct digits; guesses do nothing useful.
- Follow the clearings. Each revealed cell may carry a new clue - read it before moving on.
- Work outward from lit areas. The solve spreads from where the fog has already lifted.
- Read every clue as it appears. The fog hides the variant's markings too, so a newly lit cell may bring a cage, a line or a given you could not have known about.
Common mistakes
- Guessing to reveal cells. A wrong digit clears no fog - deduce, don't gamble.
- Assuming fog means "no clue". A fogged cell may well hold a given digit or part of a cage - it simply has not been revealed yet. The puzzle underneath is complete.
- Expecting to plan far ahead. You cannot deduce from clues you have not uncovered yet, so long-range plans across fogged ground are wasted effort.
- Ignoring the usual rules. Rows, columns and boxes all still apply as you uncover the grid.
Can I play it here?
Not yet - Fog of War Sudoku isn't one of the puzzle types SudokuStreak generates for now. But you can learn its rules above, and there's plenty here to sink your teeth into meanwhile:
- Try the closely related Killer, which you can play right now.
- Browse all the Sudoku variants we do offer.
- Or print free puzzles from the printable PDF page.
Where to play it
Fog of War Sudoku belongs to the misc family and runs on 9×9. It is not one SudokuStreak generates yet - but the deduction it trains is the deduction every Sudoku runs on.
Difficulty and type are two separate dials here: the type sets the extra rule, the level sets how much of the grid you start with. Easy Sudoku leaves 36 of the 81 cells filled, so there is always an obvious next move somewhere; Hard Sudoku leaves 26, where scanning gets you started and then leaves you stranded. Take it at the gentler end while the pattern is still new, and move up once it stops being a fight.