Nothing to sign up for, and nothing sent anywhere
Your streak, your solve times, your unfinished board and your preferences are stored in this browser on this device. There is no account because there is no server keeping your record - which also means nobody can look at it, including us.
The trade is that clearing your browser data clears your history too. Settings has an export that writes the lot to a file, which doubles as the way to carry your record to a new device.
Playable with no connection
The site caches itself on the first visit, today’s boards included, so it opens and saves normally on a plane or underground. None of the solving needs the network.
Add it to your home screen and it opens like an app, full screen, with the same puzzles. There is no separate app to download and nothing in a store.
Controls that suit the puzzle
Tap a cell to cycle sun, moon, empty. On a keyboard, the arrow keys move and space cycles; right-click or shift-space steps back the other way, which matters more here than in most puzzles because you spend a lot of time flipping a cell you placed two moves ago.
Suns and moons are distinguished by shape as well as by colour, so the board does not depend on colour vision. The grid resizes to whatever screen it is on rather than shrinking a fixed layout.
The rules, in one paragraph
Fill every cell with a sun or a moon. Never three of the same in a row, in any line. Each row and column ends up with an equal number of each. Where a small = or × sits between two cells, it means those two cells match, or that they do not.
If that is new to you, start on the daily board below - the early ones are deliberately kind - or read the full guide, which works through each deduction on a real grid.
Common questions
Is there anything to download?
No. It runs in the browser. You can add it to your home screen if you would like it to open full screen, but that is the same site rather than a separate app.
Can I tell the two symbols apart without colour?
Yes. Suns and moons differ in shape as well as colour, so the board does not rely on colour vision to be readable.
What happens if I close the tab halfway through?
The board is saved as you go and will be waiting when you come back, on the same device and browser.