Sudoku or Binary Puzzle
Sudoku is a 9×9 of digits 1 through 9. You watch rows, columns, and 3×3 boxes for the missing digit.
Binary Puzzle is 0s and 1s on 6×6 or 8×8. You watch halves, triples, and whether two lines match.
Sudoku has more symbols and a third unit, the box. Binary Puzzle has two symbols and tighter local rules: equal counts, no three identical in a line, every row and column different.
If you like placing 1 through 9, Play Sudoku. If you like a smaller grid and a yes-or-no cell, Play Binary Puzzle. Both have one solution. Neither needs a guess. How to play Sudoku and How to play Binary Puzzle are the two short pages.