The classic Dr Seuss cartoon character, The Cat in the Hat, is celebrating his fortieth birthday. And what a celebration. Meg Carter looks at the T-shirts, the games and the theme park which make the marketing of The Cat a business worth millions

If you're not familiar with either The Cat in his stovepipe hat and crooked umbrella or his creator, Dr Seuss - the man who invented the word, nerd - you soon will be. Their anniversary comes complete with a promotional bandwagon carefully crafted to launch new editions of Seuss' surreal stories and pave the way for a forthcoming BBC 2 special, two movie adaptations - one by Stephen Spielberg - and a stage musical. Oh yes, and T-shirts, cassettes and CD-Rom, fine art reproductions, board games, and, naturally, a theme park which is due to open in Orlando, Florida in 1999.

Cat helped propel Dr Seuss to the top of the children's bestseller lists. Subsequent titles including One Fish Two Fish Red Fish Blue Fish and Green Eggs and Ham contributed to worldwide sales currently estimated at 200 million and rising. That's on a par with Enid Blyton - an achievement bettered only by Tin Tin and Asterix. No mean achievement, then, for a series designed as learning books.

Dr Seuss' widow, Audrey Geisel, is in no doubt about the reason for this. "These are books children feel they own," she declares. They tell bizarre stories about strange characters - like `Yooks', `Zooks' and `Quimneys' - with detailed illustrations and quirky rhymes. And they appeal to adults who will have grown up with them themselves or who are drawn by the tales' latent morality. Take Dr Seuss' first children's book, And To Think That I Saw It On Mulberry Street - the story of a boy who's imagination is too strong for his father's taste and who grows frightened of going home because he cannot share his thoughts. A clear warning against adults stifling their kids' imaginations.

Dr Seuss' real name was Theodore Seuss Geisel - he added "Dr" to his middle name to appease his father after graduating from Oxford without a doctorate in 1926. Inspired by a report about children's illiteracy, publisher William Spaulding challenged Seuss to write Cat using just 225 words - the maximum number he thought a primary school child could absorb.

"He never truly understood where the inspiration came from, only that it was something he had to get out. He had always doodled - which was one of the reasons why he was never a very good Oxford student: he was always too distracted by his own ideas," recollects Mrs Geisel.

"When he was young he was interested in any animal with distinctly unfamiliar, inhuman form - like horns, hooves and antlers."

Seuss, who had no kids of his own, shied away from large groups. "He kept his distance from children and certainly didn't have access to the children's panels many authors writing for kids use today," she says. Instead, he buried himself in his work, spending up to a year on each book "Seuss-ifying" - to coin her phrase - memories of his childhood.

"He always felt the books were for fun and that the moment they were required reading, all the fun would be gone."

Seuss kept writing up until his death in 1991. By that time his books had been translated into 20 languages and his work was being handled by global management company ICM. "Asking why these books are still so popular is like saying `What's the secret of Shakespeare's success?" Herb Cheyette, vice president of ICM grandly declares.

Highly marketable - but only after his death did the merchandising grind into motion.

"The Seuss estate was divided into various philanthropic trusts. It became essential for the beneficiaries to reap the benefits from these properties," Cheyette explains. "We had to get into commerce to protect the copyright."

The characters were registered as trademarks - the only effective way of policing the unofficial merchandise that was being sold around the world, like bootlegged T-shirts depicting the Cat smoking a joint or flashing a peace sign. Which is why everything is now official.

Jim Henson Productions was recently licensed to produce a TV series, The Wubbulous World of Dr Seuss, featuring puppet versions of his characters in new storylines. The deal includes rights to develop the first Seuss toy range. Spielberg's film will also generate merchandise. A Seuss! Exhibition recently opened in New York offering visitors the chance to play with gadgets from the books, visit the Green Eggs and Ham kitchen and climb into the Cat in the Hat's clean-up machine. And, of course, there's the theme park. "It's attempting the impossible but it seems to be making it work," she marvels. "One ride will involve a series of two-seater wagons in a virtual reality situation."

All very tasteful, those involved insist. But one can't help wondering if someone somewhere has overlooked another reason for the books' cult status. "I like nonsense, it wakes up my brain cells. Fantasy is a necessary ingredient in living, it's a way of looking at life that enables you to laugh at life's realities," Seuss once commented. It was an observation perfectly in tune with the Sixties psyche. Besides, many of the principles latent in his book are just as relevant today. Inspired by the environmental movement in the early Seventies, he wrote The Lorax, a cautionary tale about the "Thneed" factory and the effect it had by cutting down all the "Truffala" trees. "The Lorax", a little brown creature, acted as spokesperson for the trees - and the voice of sanity.

Venture on-line and you'll get all this and more from the hoards of Nineties Seuss-obsessives jostling for attention. Numerous home pages carry tributes to the great man along with the chance to buy "Seuss-ified" underwear and slippers. You can read Seuss Trek: The Next Generation, a sci-fi comic strip. Or check out the rhyming account of the OJ Simpson Trial, as told by Dr Seuss. You can even buy a VHS copy of that movie classic Green Eggs and Hamlet. What the Cat would have made of all this mayhem, goodness knows. But one can't help suspecting he'd have been rather pleased.

A birthday collection of Dr Seuss books is published in the UK this week by HarperCollins