Squirrels.

Squirrels hide acorns in the hollows. Each sign tells how many acorns are in the run of hollows to its right or below it. Every hollow holds 1 to 9 acorns, and no number repeats within a run.

A run is the unbroken line of hollows that follows a sign, and it ends at the next trunk or at the edge of the wood. Every hollow belongs to one run across and one run down at the same time, and that is where the puzzle lives: the number you write has to suit both.

Every wood has exactly one arrangement of acorns that fits every sign. When the last hollow is right, the game notices by itself.

Moving about. Click or tap a hollow to pick it. Drag with the mouse or the finger across the wood to pick several at once, Ctrl (on a Mac Cmd) plus a click adds one to the picked hollows or takes it away, and Ctrl plus an arrow grows the picked patch. The arrow keys move from hollow to hollow and step over the edge of the wood to the other side; W A S D do the same. Escape lets the whole selection go.

Writing. 1 to 9, on the number row or the numeric keypad, or the pad under the wood, write into every picked hollow at once. The same number again takes it back. Delete or Backspace clears in one order: the numbers first, then the marks, then the colour, so one more press always takes away one more layer. A number written into a hollow with marks in it covers them; they come back when the number goes.

The four modes. The buttons under the pad say what a number writes: Normal the big number, Corner a small mark in the corners of the hollow, Centre a small mark in the middle, Colour one of nine colours. Z X C V pick one, the space bar walks round all four, N steps through the two kinds of mark and back. Holding Shift writes a corner mark and holding Ctrl a centre mark for that one number, without leaving the mode you are in.

Undo and Redo. Ctrl+Z or U goes back, Ctrl+Y, Ctrl+Shift+Z or R goes forward, as far as you like in either direction. Ctrl+R is left alone: that is the browser reloading the page. Restart takes every number, mark and colour off the wood; the clock keeps running and one Undo brings it all back.

Check and Hint. Nothing lights up while you play: finding a mistake is part of the puzzle. Check first tells you only how many numbers are wrong and which rows or columns they sit in; press it again and it marks them. Hint names the rule and the place without giving the number away, and the second press writes that one number and explains it. If something on the board is already wrong, the hint points there first. Hints and checks are free and counted, and shown when you finish. P pauses.

The clock runs from your first number to the last hollow and pauses when you leave the tab or sit still for a minute, which also hides the wood. Your progress, time, history and day streak stay in this browser, and in your account when you are signed in, so you can continue on another device. The day turns over at midnight where you are. Only the wood of the day counts towards the streak, the archive is for practice; once you are five days in, a single missed day is forgiven, at most twice a month.

Monday and Tuesday are easy (six by six), Wednesday and Thursday medium (eight by eight), Friday and Saturday hard (ten by ten) and Sunday is the challenge: twelve by twelve. The week strip under the wood takes you to the other days; the archive goes back a year, and the practice sets have eight woods of each level with no date attached.

Difficulty is measured, not guessed. Every wood is solved by a program that only knows named rules, in three layers: the plain ones (the last empty hollow of a run, a total that can be made in only one way, and the single number that both crossing runs still allow), the patterns inside a run (a number the run has to use that fits only one hollow, a pair of hollows holding two numbers between them, a total narrowed down by the crossing runs), and the hard one (writing a number in and dropping it because it runs into a contradiction). Easy and Medium never need that third layer; Sunday leans on it the most.

Short runs with extreme totals are the way in. Two hollows adding to 3 can only be 1 and 2; to 4, only 1 and 3; to 16, only 7 and 9; to 17, only 8 and 9. Three hollows adding to 6 are 1, 2 and 3; to 24, they are 7, 8 and 9.

Where two such runs cross, the hollow they share has to suit both sets, and often only one number is in both.

When a run has one empty hollow left, its number is simply the total minus what is already there. That one number then narrows the run crossing it, which is how a wood unravels.

If a number the run must use fits only one of its hollows, that is where it goes, even when that hollow could still hold other numbers on its own.