Sudoku is a 9×9 grid cut into nine 3×3 boxes. Some cells start out filled in — the dark digits are clues that never change. Your job is to fill every empty cell, and tapping one lights up the row, the column and the box it belongs to.
Rule one: each row may hold the digits 1 to 9 exactly once, with no repeats. This row already holds 3, 4, 6, 7, 8, 9, 1 and 2 — so 5 is the only digit left for the empty cell.
Rule two: each column holds 1 to 9 exactly once as well. The same empty cell sits in the highlighted column, and that column already holds 6, 1, 8, 4, 7, 9, 2 and 3 — 5 is missing here too.
Rule three: each 3×3 box holds 1 to 9 exactly once. This box holds 3, 4, 6, 7, 2, 1, 9 and 8 — and again only 5 is missing. Row, column and box agree, so 5 goes in the empty cell.
Tap an empty cell, then tap the digit you want on the keypad. The small number in the corner of a key counts how many of that digit are still missing — once all nine are on the board the key disappears. Tapping the same digit twice clears a wrong entry again.
Switch Notes on and tapping a digit pencils a small candidate into the cell instead of committing to it — tap it again to rub it out. Placing a real digit later clears that candidate from the cell's row, column and box for you.
Dark digits are clues. Green digits are your own entries, and a correct entry locks itself so a mis-tap cannot wipe it. Red means wrong: it counts as a mistake until you erase or replace it. A hint arrives in teal and blinks three times.
With a keyboard: arrow keys move the selection, 1–9 fill, N toggles notes and Backspace erases.
A wrong digit is allowed, but the Mistakes card counts it and there is no fourth: the third one ends the run. The board is locked, the clock stops and that game is not recorded on the score board.
