IN BOSTON, a Japanese restaurant hands out $10 gift certificates to patrons who can finish one before they finish their sushi. In the west of England, motorists passing a hillside near Bristol marvel at a giant one that appeared overnight. In Paris and New York, enthusiasts flock to tournaments where they can do them by the dozen. All over the world, newspapers have learned that unless they run at least one every day, they can expect to see their circulation plummet. And in bookstores everywhere, the accumulated works of human civilisation have been shoved aside to make way for yet another shipment of sudoku books.

"I've had people say that they've tried it after hearing me talk about it on the air," says John Williams, a Chicago radio host and sudoku fiend. "And then they tell me they're addicted." In the past, Williams says, he never paid much attention to ...