The rules of Stostone
Stostone — ストストーン — is a Nikoli puzzle about stones that fall.
- The bold lines divide the grid into rooms. Shade cells to build one stone in every room.
- A stone is a single connected block of shaded cells, and it never leaves its own room.
- A number in a room says how many cells that room’s stone covers. A room with no number holds a stone of any size.
- Stones in different rooms never touch side by side. Corner to corner is fine, and cells of one stone touching each other is the whole point.
- Now let the stones fall straight down, each keeping its shape. They must land filling the bottom half of the grid exactly — no gaps below the halfway line, nothing left above it.
Ways in
Count the halves first
Half the grid is shaded, and the numbered rooms already tell you part of that total. Subtract them and you know how many cells the unnumbered rooms have to supply between them, which on an easy grid often pins several of them straight away.
Every column carries half its cells
Stones fall straight down, so a cell never changes column. Since the bottom half ends up full, each column must hold exactly half its own height in shaded cells — three on a 6×6, five on a 10×10. Counting column by column is the single most useful check in the puzzle.
Walls between rooms are where stones stop
Two rooms that share a border cannot both shade cells either side of it. So a forced cell in one room clears a line of cells in its neighbour, and that is usually how a chain of deductions starts.
The bottom row must be solid
After the drop, the bottom row is completely full, so every column contains at least one shaded cell, and the lowest stone in each column has to be able to reach the floor. A column whose only candidates sit under an already-committed stone will not work.
Watch for stones that would bridge a gap
A wide, flat stone landing across a hollow leaves a hole beneath it, and holes are illegal. Before committing to a broad shape, check what will be under it when everything settles.