NOT content with pottering in the garden, or sitting back with his pipe, cardy and slippers, Wayne Gould took up a somewhat more cerebrally challenging retirement hobby than most. He is the brains behind the latest incarnation of the logic puzzle that has got people chewing their pens the world over.

Coming from the Japanese words su, meaning number, and doku, roughly translating as single or unique, you could be forgiven for thinking that Sudoku, the grid-fill number game, originated in Japan.  However, Wayne Gould, aka Pappocom, says, "very little is known about its history. It emerged in the United States in the late 1970s -so some people assume that it was invented there, but that doesn't necessarily follow. I think it was probably posed by some academic mathematician, over the course of the 200 years since Euler popularised Latin Squares."

Swiss mathematician Leonhard Euler's Latin Squares comprised grids in which every number or symbol appears once in each row or column. More than two centuries later, the difference for Sudoku players is that the grid is subdivided into blocks of nine, in which the numbers from one to nine must appear in each block, as well as each row and column of the grid.

Sounds like some fiendishly difficult maths homework? Think again. In fact, no maths skills are required to complete the puzzle; it is only incidental that numbers are used at all, these could be replaced by anything such as letters, symbols or colours.

Gould, a New Zealander, first encountered the puzzle in Tokyo, Japan in 1997 after retiring as a judge in Hong Kong. "I saw a book of puzzles in a bookshop and bought the book. Later, I worked out what the puzzle was and what you were expected to do," he says. "Like a crossword grid, I thought the empty squares were just begging to be filled in. However, you don't need any language skills at all, you just need logic. The `rules' are so simple, and you can start playing at once - but the reward (compared to the effort) is high."

And thus he was hooked. The next six years were spent developing a computer to generate the puzzles.  "I had just retired when I discovered (or `uncovered') Sudoku, and I had been looking forward to brushing up my computer-programming skills. So Sudoku became my first post-retirement programming project."  Not having a specific background in mathematics, Gould says: "I had to teach myself all the maths skills I needed as I went along."

This "post-retirement project" is now a thriving enterprise. Poppacom produces puzzles for "55 countries worldwide, in 29 languages (not that language matters!)".

Sudoku is probably most popular in the United Kingdom at the moment, "because they got started first (in the `modern era')," says Gould. Indeed, the United Kingdom seems to be in full grip of the craze. The Observer newspaper reported that, "If the first week of May 2005 will be remembered for a general election, the second will go down as the week of Sudoku," referring to the launch of the puzzle, in which newspapers, websites and TV stations were scrambling over each other to give coverage to the new mind bending obession to sweep the nation. The puzzle also manifested itself in literally gargantuan proportions in the British countryside, when the largest Sudoku puzzle ever was created on a hillside as part of an advertising campaign for a game show. The 81 sq m board was cleverly constructed near a piece of motorway notorious for heavy traffic, meaning that drivers proceeding at a snail's pace had time to solve the puzzle, no doubt providing a welcome relief from the hundredth game of Eye Spy and the repeated cries of "are we nearly there yet?".

But Gould says other countries are catching up. "We are on all six continents, so there are a lot of countries that are very keen." Bulgaria now numbers among these with The Sofia Echo's launch of the puzzle earlier this month, giving you the chance to try the brainteasing sensation for yourself. But be warned, Gould says of his family: "They are all hopelessly hooked - and have been for a long time." Pens at the ready and turn to our back page...