One might expect a game of logic to appeal to very few people--mathematicians, maybe, computer geeks, compulsive gamblers. Yet in a very short time, Sudoku has become extraordinarily popular, bringing to mind the Rubik's cube craze of the early 1980s.
Unlike the three-dimensional Rubik's cube, a Sudoku puzzle is a flat, square grid. Typically it contains 81 cells (nine rows and nine columns) and is divided into nine smaller squares containing nine cells each; call them subgrids. The game begins with numbers already printed in some cells. The player must fill in the empty cells with the numbers 1 to 9 in such a way that no digit appears twice in the same row, column or subgrid. Each puzzle has one unique solution.
This article was originally published with the title The Science behind Sudoku.