Fill in the blanks so no number repeats within any row, any column, or any of the small boxes marked off by bold lines.
There's exactly one rule — the same number can never appear twice in the same row, the same column, or the same box. Pick a blank cell and tap a number button to fill it in; if it breaks the rule, it turns red.
"Sudoku" is short for a Japanese phrase meaning "the numbers must occur only once," though the rule itself actually comes from an old mathematical idea called a Latin square. Remarkably, every puzzle has exactly one solution, so you have to nail down each cell through logic rather than guessing.
If you get stuck, start by ruling out numbers that already appear in a given row or box. Once only one possible number is left for a cell, that's your answer. This process of narrowing down the candidates is the heart of solving sudoku.