In 1997, Wayne Gould, a man from New Zealand, was visiting Tokyo. While he was browsing a bookstore, he saw the squares and felt tempted to fill them in. Over the next six years, he developed a computer program that instantly makes up Sudoku puzzles.

Gould's wife published one of his puzzles in the local newspaper. It spread to Britain and was published in the Times, where it took off.

Sudoku has been identified as a classic meme - a mental virus that spreads from person to person across national boundaries. The puzzle is using our brains to multiply across the world.

Sudoku consists of 81 squares, divided into nine blocks of nine squares each. Some squa...