Launched yesterday in the City section, Shikaku - also known as 'rectangles' - involves dividing a grid-like board into small rectangular or square boxes. Each box must contain a number that indicates the area of the box (detailed instructions at promotions.scmp.com/shikaku). Beginners to this puzzle may start with a guess, but soon realise that the fun lies in searching for rectangles using logic.

Just like its more popular sibling, Shikaku uses only numbers, rather than letters, like a crossword. Anyone can solve and enjoy it. 'These puzzles speak the same language, because numbers are universal and can be understood by everyone no matter where they live,' says 61-year-old Kaji.

'My policy is that puzzles are entertainment. They should be relaxing rather than challenging or educational as such. Laughing and humour are important.'

Puzzles, he says, are a spa for the mind. 'I don't see the point of making such a difficult puzzle that nobody can solve or enjoy it. I have always desired to create a puzzle that can be enjoyed by many people around the world, so that they can have a break from daily stress. This has been my company's goal [from the outset].'

Nikoli, the company he founded in 1980, has about 30 employees and has churned out about 300 puzzles since Sudoku began in Japan in 1984. Though known as the Godfather of Sudoku, Kaji, a college dropout, acknowledges that he got the idea from a game called 'Number Place' in a US puzzle magazine.

Ironically it's not these brain games that the low-key Kaji credits for his good health; instead, he says his secret is smoking, drinking, and betting on racehorses. (Nikoli is named after the winner of the Irish 2,000 Guineas in 1980.)

But Kaji's puzzles are undoubtedly helping many preserve their mental sharpness. Sudoku alone appears in more than 600 newspapers, on thousands of websites and in dozens of books in more than 70 countries, played by people aged five to 90.

Many studies have shown the link between such mentally stimulating activities and halting the advancement of dementia.

One study, published in Neurology in August 2009, tracked nearly 500 people aged 75 to 85 who did not have dementia at the start of the study for an average of five years. It was found that for every brain activity a person participated in - reading, writing, doing crosswords, playing board or card games, having group discussions, or playing music - the onset of rapid memory loss was delayed by 0.18 years.

Another study published last November in BMC Medicine observed dementia patients from five nursing homes in Germany. Half acted as a control group, while the other half were put through a regime of mental and behavioural exercises that included games (bowling, croquet, balancing exercise), puzzles, 'daily living' activities (preparing snacks, gardening, crafts) and a 'spiritual element'.

After 12 months of therapy, the group maintained their level on the Alzheimer's Disease Assessment Scale and their ability to carry out activities of daily living, while the control group all showed a decrease in cognitive and functional ability.

Dementia is a degenerative condition that can progress fast or slowly, and has a variety of causes. Symptoms include confusion, loss of memory, and problems with speech and understanding. The condition can be unpleasant for both the sufferers and their families.

With the brain, it's use it or lose it. And it's never too late to start.

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Number, in millions, of seniors with dementia projected in the world by 2050, according to a Shanghai-based study on mental activity