Beavers.

1. Every tree gets one beaver lodge right beside it, above, below, left or right. Each lodge belongs to one tree only, so there are as many lodges as trees.

2. Lodges never touch each other, not side by side and not corner to corner.

3. The number by each row and each column says how many lodges that line holds.

Exactly one arrangement fits. Grass is your own note that no lodge goes in a cell; only the lodges count.

Tap or click a cell: once for grass, a small dot that means no lodge here; twice for a lodge; three times to clear it. Hold the button or your finger and drag to sweep the same mark over many empty cells at once. Trees never change. Nothing lights up while you play: finding a mistake is part of the puzzle.

Check first tells you only how many marks are wrong and roughly where. Press it again and it shows which. Hint first tells you where to look; press it again to see the step marked and filled in. Hints and checks are free, counted and shown when you finish. Clear empties the pond; the clock keeps running, and Undo brings it all back.

Keyboard: arrows move, Space or Enter cycles the cell, X grass, B lodge, Delete or Backspace clears, U or Ctrl+Z undoes, R or Ctrl+Shift+Z redoes, P pauses.

The clock runs from your first move to the last cell and pauses when you leave the tab, which also hides the pond. 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.

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

Difficulty is measured, not guessed: every pond is solved by a program that uses only steps a person would use, and the candidates for a date are ranked by how many of the harder steps they need. Monday to Saturday never need a guess. Sunday may ask you to try one cell and follow it until a rule breaks, never more than eight steps away; the Hint shows that step too, with the first steps of the chain marked on the pond.

A cell with no tree right beside it is always grass, and so is the rest of a line once its number is reached.

A tree with only one free cell beside it has its lodge there, and then the eight cells around that lodge are grass.

When a tree's lodge can only go in one of two cells, any cell that touches both of them is grass.

Try every way to fit the lodges a line still needs, never two side by side. A cell that every way uses is a lodge; a cell that no way uses is grass.

When every way to fit a line's lodges touches the same cell of the next line, that cell is grass.