Foxes.

A family of foxes shares one lair, split into chambers, and one thing has gone missing. Every fox is somewhere in the lair, and so is the lost thing.

1. Every row and every column holds exactly one of them: a fox or the lost thing. Nobody can be on a stone.

2. Every numbered clue is true, and together they leave exactly one way to place everyone.

3. The fox who ends up alone in a chamber with the lost thing has it. Find out who, and in which chamber.

Nothing has to be guessed: every fox can be placed by reasoning from the clues.

Tap a cell, then tap a fox below the board to put it there. Tap Nobody for a cross where no one can be. Switch on Notes to write a fox's letter in the cells where it could still be; drag across cells to pick several at once. A fox stands on the board only once: put it somewhere else and it moves, as one step you can take back with a single Undo.

On a keyboard, arrows move, the numbers place the foxes and the lost thing, Shift and a number writes a note, X puts a cross, Delete clears, U undoes, R redoes, H asks for a hint, C checks, P pauses. A number typed with several cells picked goes in as a note, because one fox can only be in one place.

On a phone the clue list sits above the board, three lines at a time; the reader under the board shows one clue with arrows to the next, and a tap on it strikes it out. Every fox's trait is on its button under the board, and Nobody sits right after the last one.

Check first tells you how many of your foxes and crosses are wrong and roughly where; press it again and it marks them. Notes are never judged. Hint names the clue and the place, then takes one step and writes down everything that step shows: a fox, its notes, or crosses. When a clue comes round again for the same fox, Hint says so and follows it on through the clues it leads to in the same press. Hints and checks are free, counted and shown when you finish. Clear empties the board; the clock keeps running and one Undo brings it back.

The clock pauses when you leave the tab or sit still for a minute, which also hides the board but never the clues. Your progress, time, history and day streak stay in this browser, and in your account when you are signed in.

Monday and Tuesday are easy, five by five with four foxes, and every clue can be read on its own. Wednesday and Thursday are medium, six by six with five foxes, and clues start to link two foxes. Friday and Saturday are hard, seven by seven with six foxes. Sunday is the challenge, also seven by seven: one to three times a lair, one fox has to be tried in one of its two cells and followed, within six steps, until something breaks.

The week strip under the board takes you to the other days; the archive goes back a year, and the practice sets have twenty-four lairs of each level, in sets of eight, with no date attached.

Read a clue about one fox and write its letter as notes wherever it can still be. A fox with only one cell left goes there.

A row with room for just one fox, or with just one open cell, settles itself; so does a column.

A clue about two foxes rules out every cell where the other one could not fit.

Once you know the chamber with the lost thing, only one fox can be there with it, so nobody else can.