Swans.

Draw one loop through every swan. At a white swan the loop goes straight and turns in the cell just before or just after. At a black swan the loop turns and goes straight through both neighbouring cells.

The loop runs from the middle of one cell to the middle of the next, up and down or left and right, never diagonally. It is a single closed ring: it never crosses itself, never branches, and never passes through the same cell twice. Cells with no swan on them may be left out of the loop altogether.

When every swan has the shape it asks for and the ring is closed, the lake is finished and the game notices by itself.

Drag from cell to cell and the loop follows your finger; drag back along what you have drawn and it rubs out again. If you would rather not drag, tap one cell and then a neighbouring one, and the step between them is drawn; tap the same cell twice to stop. Tapping the gap between two cells goes through the three states on its own: a line, then a cross, then nothing. The cross is only your own note that the loop does not run there; it is never checked and never required.

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, and the second press 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 lake; the clock keeps running and one Undo brings everything back.

Keyboard: arrows move the cursor from cell to cell, Enter starts drawing and the arrows then extend the loop, Enter again or Escape stops. Space goes through line, cross and nothing on the direction you used last, X crosses it, Delete clears it, U or Ctrl+Z undo, R or Ctrl+R redo, P pauses.

The clock runs from your first line to the last one and pauses when you leave the tab or sit still for a minute, which also hides the lake. 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.

A lake finished with no hint and no check counts as clean; your history keeps count. Only the lake of the day builds the streak, and after five days in a row one missed day is forgiven, at most twice in a month. When you finish, Share copies one line with the date, your time and how much help you took. It never copies the lake or your loop, so nothing is spoiled for anyone.

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

Difficulty is measured, not guessed. Every lake is solved by a program that only knows named rules, in three layers: what one cell or one swan settles on its own, the patterns between neighbouring swans, and the hard ones, where a step is tried out and ruled out because it would close the loop too early or end in a contradiction. Easy and Medium never need that third layer; Sunday leans on it the most.

Start with the black swans at the edge. A black swan turns, so it can never send an arm off the board, and a black swan in a corner has only one turn left to make.

A white swan against the edge has to run along that edge, because the loop goes straight through it and the other way is off the board.

Every arm of a black swan runs straight on through the next cell, so a single black swan usually settles five steps at once. That is the cheapest opening on any lake.

Three white swans in a row cannot all be crossed lengthways: the middle one would have no turn beside it, so the loop cuts across the row instead.