Summary: A Mosaic puzzle is a picture hidden inside a grid of numbers. Each number counts the shaded
cells in its own 3×3 block — itself included — and shading every cell the numbers demand reveals the
picture. This guide covers the rule, the two deductions that solve any board, and how to play one online.
Overview
You get a grid of cells — square or rectangular. Some cells carry a number from 0 to 9; the rest are
unnumbered. The number tells
you how many cells in that cell’s own 3×3 block end up shaded — the numbered cell itself plus the
up-to-8 cells touching it. Work out which cells those are, shade them, and a picture appears.
If you have ever played Minesweeper, you already know the counting rule: a number describes its own 3×3
neighbourhood. The goal here is the opposite, though. In Minesweeper you avoid what the numbers count; in
a Mosaic puzzle you shade it on purpose, because the shaded cells are the picture.
The Goal
Decide, for every single cell, whether it is shaded or not, so that every number is satisfied. Exactly
one arrangement does that, and it is the picture.
The Rules
- A number counts its own block, including itself. This is the rule people get wrong first. A 3 does not mean “three of my neighbours” — it means three cells out of the nine-cell block centred on me, and I might be one of them.
- Blocks overlap. Every cell sits at the centre of its own block, so neighbouring numbers describe heavily overlapping regions. They are not the separate, non-touching boxes of a Sudoku grid — the overlap is exactly what lets one number narrow down another.
- Numbered cells get shaded too. The number is the given, not the cell. A cell with a 5 printed in it may well be part of the picture. In the online and interactive-PDF versions the number shows through the shading, so you never lose it. The printed answer key is the one place the numbers disappear entirely: it drops them so the picture reads cleanly, which is why the solution above is a plain silhouette.
- Blocks are smaller at the edges. In the middle of the board a block is 9 cells. Along an edge it is 6. In a corner it is 4. So the largest number you can ever see in a corner is 4, on an edge 6, and in the middle 9.
- An unnumbered cell is not an empty cell. A cell with no number printed in it is simply a cell you have not been told about — it is shaded or unshaded like any other, and the surrounding numbers decide which.
- The shaded cells do not have to connect. Nothing requires the picture to be one piece. Islands and gaps are normal.
- One solution, and you never have to guess. Every puzzle is built so that pure reasoning gets you all the way there. If you are guessing, there is a deduction you have not spotted yet.
The Two Deductions That Solve Any Board
Everything in a Mosaic puzzle comes from two observations, applied over and over. It helps to mark cells
you have ruled out — a light dot or a cross — because a cell you know stays unshaded is what pushes the
next number over the line.
- The number is already satisfied → the rest of its block stays unshaded. Once a block holds as many shaded cells as its number, every still-undecided cell in that block must stay unshaded. The extreme case is a 0: it satisfies itself immediately, so its whole block is unshaded.
- The number needs everything that is left → shade all of it. If the shaded cells plus the still-undecided cells in a block add up to exactly the number, every undecided cell has to be shaded. The extreme case is a number equal to its block size — 9 in the middle, 6 on an edge, 4 in a corner — which shades the whole block on sight.
How to Start Solving
Work through the small board above. Rows are numbered 1–5 from the top, columns 1–5 from the left, so
r3c2 means row 3, column 2. Fair warning about a board this small: its “picture” is only a solid
block. Recognisable shapes need room, which is what the bigger grids are for — the method is identical.
- Sweep the zeros first. They are free information and they clear space fast. The whole of column 1 is zeros, and each one blanks its own block, so columns 1 and 2 are unshaded top to bottom. The 0 at r2c4 is the one doing the heavy lifting on the other side: its block covers rows 1–3 across columns 3–5, which blanks the entire top-right. Between them, rows 1–3 and columns 1–2 are all ruled out — only the six cells in rows 4–5, columns 3–5 are still open.
- Find a number with just one candidate left (deduction 2). The 1 at r3c2 has a nine-cell block covering rows 2–4, columns 1–3. Eight of those nine are already ruled out; the only cell still undecided is r4c3. The clue needs one shaded cell and there is exactly one candidate, so r4c3 is shaded.
- Do it again (deduction 2). The 2 at r5c2 covers rows 4–5, columns 1–3. Columns 1–2 are blank, so its only candidates are r4c3 — now known shaded — and r5c3. One shaded plus one undecided makes two, which is exactly the clue, so r5c3 is shaded too.
- Cash in a block-size number. The 6 at r5c4 sits on the bottom edge, so its block is only six cells: rows 4–5, columns 3–5. The number equals the block size, so all six shade, and the board is done.
- Confirm with the rest. The 4 at r5c5 is in a corner, so its four-cell block is now fully shaded — correct. The 6 at r4c4 has a nine-cell block holding exactly the six shaded cells — correct. The 2 at r3c5 sees exactly two — correct. Every number agrees, so you are done.
Two honest notes about that walkthrough. First, this board has a shortcut: step 4 alone would have
finished it straight after the zeros, because that 6 covers every remaining cell. Steps 2 and 3 are there
because the general move — counting a clue’s shaded and undecided cells and finding they add up — is what
you will actually use on a full-size board, where block-size numbers are rare luxuries. Second, a board
this bare never needs deduction 1 in its everyday form: after the zeros there is nothing left to blank.
On a 10 × 10 it is the workhorse, and you will be closing off satisfied clues constantly.
The method never changes with size — only how many turns of the crank it takes. Each cell you settle
changes the count in every block that covers it — up to nine of them, fewer near an edge or corner —
which usually settles another number, and so on.
What Changes From Puzzle to Puzzle
- Grid size, 5×5 to 40×40. A small board is a quick warm-up; a 15×15 gives the picture enough room to read as a real shape, and the largest boards carry real detail — at the cost of a longer solve and a page big enough to print them legibly.
- How many numbers are printed. This is the difficulty setting, and it is the only thing that changes between an Easy and an Expert board — the picture itself is unaffected. Easy leaves the grid densely covered in numbers, Medium prints one on about half the cells, and Hard and Expert strip out more and more, until on Expert only about a quarter of the cells carry a number and you are leaning hard on the cells you have ruled out. What never changes is the promise: every level is still solvable by pure reasoning, with one answer. An Expert board is barer, not riskier.
- The picture. A puzzle’s answer can come from artwork the person making the puzzle supplies — a heart, an animal, a seasonal shape — or from the software’s own picture generator, which produces an abstract pattern rather than a recognisable object. That is why some Mosaic books are themed and others are pure pattern.
Play It Online
Mosaic puzzles can also be published to play in the browser — that lane comes from the Interactive
add-on, so whether you meet one online is up to the publisher. Clicking a cell cycles it through three
states: unshaded → shaded → ruled out → back to unshaded. Nothing is locked, so numbered cells shade like
any other, and the number stays visible on top.
The ruled out mark is your pencil note, and the two checks read it differently. Verify ignores it
entirely — it counts only shaded cells, so ruling out changes nothing there. The live highlight, which
flags any number whose block already holds too many shaded cells or can no longer reach its total, treats
a ruled-out cell as one you have decided stays blank. So your own marks can light up a clue that the same
board would not flag with those cells left plain. Useful when your marks are right; worth undoing a mark
before you trust the warning. The general player controls are covered once in
How to Play Puzzles Online.
Outcome
You now know the one rule that defines a Mosaic puzzle — every number counts the shaded cells in its own
3×3 block, itself included — the two deductions that solve any board, why zeros and block-size numbers
are the places to start, and what changes between an easy board and a hard one. Want to make your own,
with your own pictures? See
How to Create Mosaic Puzzles in Puzzle Maker Pro.
FAQ
Does the number count the cell it is printed in?
Yes, and this is the single most common mistake. A number counts the shaded cells in the 3×3 block
centred on itself, and the numbered cell is one of those nine. A 1 with no shaded neighbours means the
numbered cell itself is the shaded one.
Can a cell with a number in it be shaded?
Yes. The number is a printed hint, not a locked cell — a numbered cell is part of the picture as often as
any other. In the online and interactive-PDF versions the number shows through the shading; the printed
answer key is the one place it drops away, so the picture shows on its own.
What is the biggest number I can see?
Nine in the middle of the board, six along an edge, four in a corner — the block runs out of cells at the
border. That also makes those numbers powerful: a 4 in a corner shades its whole block immediately.
Can a Mosaic puzzle have more than one solution?
No. Every puzzle is generated so that pure step-by-step deduction reaches exactly one answer, which is
also what guarantees you never have to guess.
Do the shaded cells always form a connected shape?
No. Nothing in the rules forces the picture to be a single piece — separate islands and holes are normal
and often part of the image.
How is it different from a nonogram?
Both end in a picture, but the clues work differently. A nonogram numbers the outside of the grid and
describes runs of shaded cells along a whole row or column. A Mosaic puzzle prints its numbers inside the
grid, and each one only ever talks about the 3×3 block around itself — so you reason locally, cell by
cell, instead of fitting runs into lines.