How to play Otters.

1. Draw one closed loop along the grid lines. The otters need a single river: no branches, no crossings, no second loop. A grid with five patches by five has six dots by six; the size changes through the week, from five by five on Monday to nine by nine on Sunday.

2. A number in a patch counts its sides. It says how many of that patch's four sides the river runs along, from 0 to 3. A patch with no number says nothing at all.

3. A cross is your own note that a side stays dry. Crosses are yours; the game does not judge them, except when you ask with Check. When the loop closes and every number agrees, the game notices by itself.

Crosses are notes, not answers. Placing a cross never turns anything red while you play; only Check judges, and only when you ask. Tap a side to draw it, tap again for a cross, once more to clear. On a keyboard, arrows move the cursor, Ctrl plus arrow draws, Shift plus arrow crosses, P pauses.

A cross you were wrong about is shown by Check, never by the board turning red while you play.

Every river opens with steps like these. Hint finds the next one for you and says in words which rule and which place it means, without pointing at anything; press it again and it rings those sides and draws them. Hints are counted and shown when you finish, never hidden.

• A 0 or a 3 in the corner. A 0 crosses all four of its sides; a 3 in a corner cannot send the river back on itself, so both outer sides are river.
• Two lines at a dot. Every dot the river reaches has exactly two lines; as soon as two meet, the rest of that dot's sides are crossed.
• A number that already has enough, or needs all it has left. Once a number's sides carry as many lines as it asks for, the rest are crossed; once exactly as many sides are open as it still needs, all of them are river.
• Marks already placed. Crosses and lines you placed narrow what still fits; Hint reasons from them the same way it reasons from the numbers alone.