Otters.

Draw one closed loop along the grid lines. A number in a patch of marsh says how many of its sides the loop runs along. A cross is your own note that a side is not part of the loop.

The simplest examples: a 0 has none of its four sides in the loop, and a 3 in a corner of the marsh has both of its outer sides in the loop (a corner patch only touches two sides of the grid to begin with). A patch with no number can have any number of sides in the loop, from none to all four.

Every dot the loop reaches has exactly two lines meeting it, one in and one out. Crosses are never checked and never required; the game notices by itself once the loop closes and every number agrees.

Tap or click a side to draw it: the first tap draws the river, the second tap puts a cross, the third tap clears it. 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 names the rule and the place in words only, and the second press rings those sides and draws that one step for you. 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. Clear empties the marsh; the clock keeps running and one Undo brings everything back.

Keyboard: arrows move the cursor from dot to dot, Ctrl plus arrow draws or removes the river in that direction, Shift plus arrow puts a cross there, Enter starts and ends a run you steer with the arrows, Escape cancels it, X crosses the last direction, Delete clears it, U or Ctrl+Z undo, R or Ctrl+R redo, P pauses.

The clock runs from your first move to the closing of the loop and pauses when you leave the tab or sit still for a minute, which also hides the marsh. 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 (five by five), Wednesday and Thursday medium (six by six), Friday hard (seven by seven), Saturday tough (eight by eight) and Sunday is the challenge: nine by nine. The week strip under the marsh takes you to the other days; the archive goes back a year, and the practice sets have eight rivers of each level with no date attached.

Difficulty is measured, not guessed. Every river is solved by a program that only knows named rules, in three layers: the local ones (a 0, a 3 in a corner, a dot that already has two lines), the patterns between numbers (two 3s side by side, a 3 next to a 0, a 1 in a corner), and the hard ones (a side that would close the river too early, and trying a side out for one step). Easy and Medium never need that third layer; Sunday leans on it the most.

A 0 is the best friend you have: the river runs along none of its four sides, and those crosses often decide the patches around it.

A 3 in the corner of the marsh cannot send the river back on itself, so both of its outer sides are river. Two 3s side by side always use the side between them, and their outer sides too.

Every dot carries either no line or exactly two. As soon as two lines meet at a dot, the rest of its sides are crossed; as soon as three sides are crossed, the fourth is crossed too.

Near the end, watch for a side that would close a small loop before every number is satisfied. It cannot be river, so it is crossed.