How to play Boo Who?

Boo Who? is a cozy logic puzzle: sudoku's one-per-row-and-column rule, plus a tiny ghostly whodunit. Boo got the fright of the afterlife, and one of the Sweet Spirits did it. Place every ghost on the map so that every clue is true – the ghost left alone with Boo is the culprit.

The goal

Place every ghost on the map so that every clue holds. Then name the culprit: the one ghost who is alone with Boo in a room. Every case has exactly one solution, and you can always reach it by logic alone – you never have to guess.

The map

Each case is a 6 × 6 map of a haunted house, split into rooms. Some squares hold an object – a piano, a mirror, a fern – and a ghost may stand on it. Some squares are blocked, and no ghost can stand there.

One ghost per row and column

No two ghosts ever share a row or a column. You don't need to keep track of this yourself: a placed ghost crosses out the rest of their row and column, and if you try to put someone there, that ghost pipes up – My row! – until you move them. There are no wrong moves, only clues that don't hold yet.

The clues

Every clue is true in the solution. They come in a few kinds:

Where a ghost was: in a room, like Mochi was in the Kitchen., or in a certain row or column.
At or beside an object: Mochi was at the piano. means on the piano's own square. Biscuit was beside the teapot. means on a square next to it – see what beside means.
Compared with another ghost: beside them, in the same room, or higher, lower, further left or further right. Toffee was higher up than Waffles. means Toffee's row is anywhere above Waffles' row, not only straight above. From Boo, Pudding was 2 rows up. counts the rows exactly, and says nothing about the column. From Pudding, Boo was 1 row down and 2 columns left. leads to one square.
Next to a wall: on the outer edge of the map, or next to a blocked square. The border between two rooms is not a wall.
Company: Mochi was alone with Biscuit. means those two were the only ghosts in their room. No one else shared Toffee's room. means nobody else was in Toffee's room. Some clues count how many ghosts were in a room.
Not: some of these can be turned around, like Waffles was not in the Kitchen.

Next to each clue, the game shows how it stands on your board right now: ✓ it holds, ✗ the board breaks it, · not settled yet. A ✗ means some ghost is in the wrong place.

“Beside” means the same room

Beside means on a neighbouring square – up, down, left or right, never diagonally – and in the same room. Two squares that touch across a room border are not beside each other. It is the rule that is easiest to miss.

Beside the teapot: only the two squares next to it inside the Kitchen count (✓). The Parlour square to its left and the Garden square below it touch the teapot, but they are in other rooms (✗).

A worked example

This is the tutorial's case, the Gatehouse: a small 4 × 4 map with three rooms – the Parlour, the Kitchen and the Garden – and three objects: a piano, a teapot and a fern. Four ghosts need a place: Mochi, Biscuit, Pudding and Boo. The clues:

Mochi was at the piano.
Biscuit was beside the teapot.
Boo was beside the fern.

The Gatehouse, before any ghost is placed.

Clue 1 puts Mochi on the piano's square. Mochi now has that row and that column to themselves: no other ghost can use them.
Clue 2: only two squares are beside the teapot, both in the Kitchen. One of them is in Mochi's row, so Biscuit takes the other.
Clue 3: the fern sits in the Garden's corner, with two squares beside it. One is in Mochi's column, so Boo takes the other.
Pudding has no clue, but only one row and one column are still free. Pudding goes where they cross.
Every clue holds. Boo and Pudding share the Garden, and no one else is there: Pudding did it. Spook solved!

Solved: Mochi at the piano, Biscuit beside the teapot, Boo beside the fern – and Pudding alone with Boo in the Garden.

Tips for your first cases

Start with the clues that point at a single square, like at the piano.
After each ghost you place, look at the row and column it took. They often leave another ghost only one place to go.
Mark the squares a ghost can't be on: long-press a square on a phone, or switch to Rule out on a computer.
A ghost with no clue of their own often goes last, on the only free row and column.
Undo is free. Try an idea, and take it back if a clue turns to ✗.

Ready?

A new case is waiting every day at midnight, your local time. On your first visit the game offers a two-minute tutorial with a guide ghost, and the “?” button next to the case number opens it again any time.

Play today's case