Pick a cell, then press a digit — on the pad, or on your keyboard. The same digit again rubs it out.
Notes writes small pencil marks instead. N or Space switches between the two, and a digit you write clears itself out of the pencil marks it contradicts.
Arrow keys move the cursor. Backspace clears a cell, U undoes, C checks and H hints.
A digit that repeats in its row, column or box turns red on its own, without being asked.
Check marks every digit that is simply wrong — the ones that break no rule you can see, and would otherwise sit there quietly until the grid stopped adding up an hour later. It says which squares, never what belongs in them.
Hint fills one square in and fixes it like a printed digit, so it is something you can build on rather than another maybe. It takes the cell you have selected, or picks one if you have not.
One puzzle a day, and it keeps your place. New grid at midnight UTC.
The sticker at the top is the day's grade, and it is drawn fresh every
morning rather than following a weekly ramp. It means two specific
things: how many digits you are given to start with, and how hard the
hardest necessary deduction is. Nothing above Hard is ever set,
and no puzzle here has ever needed a guess.