Hares.

Fill the meadow so that every row, column and block has each number once. Hares leap like a chess knight: two burrows a knight's leap apart never hold the same number. From Friday on, hares also refuse to sit next to the same number in any direction, corners included.

A knight's leap is two burrows one way and one across, the move a knight makes on a chessboard. It reaches over rows, columns and blocks alike, which is where the puzzle differs from the number square you already know: a burrow far away can still rule a number out.

The numbers the meadow gives you are heavier and sit a shade darker. Every meadow has exactly one filling that fits every rule. When the last burrow is right, the game notices by itself.

Moving. Tap or click a burrow to select it; drag across the meadow and every burrow you cross joins the selection. Ctrl (on a Mac Cmd) plus a click adds one burrow or takes it out again. The arrow keys, or W A S D, move the selection and wrap round the edge; Ctrl plus an arrow widens it. Escape lets the selection go. A burrow the meadow gave you can be selected, for the marks and colours around it, but never overwritten.

Writing. The keys 1 to 9, the numeric keypad and the pad under the meadow all write the same way, into every selected burrow at once. The same number again takes it back, so the way out is always one press. Delete or Backspace clears in one order: the big numbers first, then the marks of those burrows, then the colour. A number written into a burrow that already carries marks simply covers them; take the number out and they are all back.

Four modes. The four buttons under the pad say what a number writes: Normal the big number, Corner a small mark in the corners, Centre a small mark in the middle, Colour one of nine colours over the burrow. Keys Z X C V pick one, the spacebar cycles through them, N goes Corner, Centre, Normal, and holding Shift borrows Corner or Ctrl Centre for as long as you hold it. Nothing lights up while you play: finding a mistake is part of the puzzle.

Undo, Redo and Restart. U or Ctrl+Z go back, R, Ctrl+Y or Ctrl+Shift+Z forward, as far as you like; Ctrl+R stays your browser's reload, we never take it. Restart puts the meadow back to the numbers it was given and clears everything else, colours included; the clock keeps running and one Undo brings it all back. P pauses.

Check and Hint. 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. With exactly one burrow selected it reads only that row, column and block. 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.

The clock runs from your first number to the last burrow and pauses when you leave the tab or sit still for a minute, which also hides the meadow. 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 meadow 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, blocks of two rows by three columns, numbers 1 to 6, with the knight's leap. Wednesday and Thursday are medium and Friday and Saturday hard, both nine by nine with blocks of three by three; from Friday the touching rule joins in and fewer numbers are given away. Sunday is the challenge, the same nine by nine with both extra rules and the fewest numbers to start from. The week strip under the meadow takes you to the other days; the archive goes back a year, and the practice sets have eight meadows of each level with no date attached.

Difficulty is measured, not guessed. Every meadow is solved by a program that only knows named rules, in three layers: the plain ones (a burrow with a single number left once its row, its column, its block and every hare within reach have taken theirs, and a number that fits only one burrow of a row, a column or a block), the patterns (a number held in place by the knight's leap or the touching rule alone, a pair of burrows holding two numbers between them, a number confined to one line inside a block), and the hard one (writing a number in, following it one step and dropping it because it runs into a contradiction). Easy and Medium never need that third layer; Sunday leans on it the most.

Start where the meadow is crowded. A block with six or seven numbers in it usually gives the rest away, and every number you write there narrows two more lines at once.

Then use the leap. When a row is missing a number and two of its empty burrows sit a knight's leap from that same number elsewhere, the third burrow is the only one left. That step is the heart of this puzzle, and the classic number square has nothing like it.

On the king days, look at pairs of touching burrows first. Two burrows side by side or corner to corner cannot both hold the same number, which quietly halves a lot of choices around a block edge.

When a number can only sit in one row of a block, it is gone from the rest of that row everywhere else on the board. That is often the step that unlocks a stubborn corner.