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.