Sudoku
Sudoku is a number-placement puzzle on a nine-by-nine grid in which every row, every column and every three-by-three box must contain each of the digits 1 to 9 exactly once.
No arithmetic is involved at any point. The digits are simply nine distinct symbols, and the puzzle would work identically with nine colours or nine letters. Some of the squares arrive already filled — the givens — and a properly made sudoku has exactly one solution reachable by logic, with no guessing required.
The basic techniques have names. A naked single is a square where eight of the nine digits are already excluded by its row, column and box. A hidden single is a digit that has only one legal square left in some row, column or box. Beyond those, solvers use pointing pairs, box-line reduction and patterns like the X-wing, which eliminate candidates without placing anything.
The modern puzzle appeared as Number Place in Dell Magazines in 1979, designed by Howard Garns. Nikoli published it in Japan in 1984 under the name it kept, and it reached the rest of the world in 2004. Mathematicians proved in 2012 that no sudoku with only sixteen givens can have a unique solution, so seventeen is the floor.