Just over four decades ago, a small boy asked his father what a tickle looked like. And the rest, as they say, is history.

The question inspired Cleckheaton-born Roger Hargreaves to draw his six-year-old son an orange man with long wiggly arms, a big cheesy grin and a little blue hat. And with that simple sketch, the first of the Mr Men characters, Mr Tickle, was born.

On August 10, 1971 – 40 years ago tomorrow – the first six Mr Men books were published. The hilarious stories with their simple, bold illustrations were instantly a huge success, going on to sell more than 100 million copies across the globe, and later becoming a children’s television series.

Sadly, Roger Hargreaves died in 1988, and so was unable to fully realise the enduring appeal of his creation, which continues to delight children and adults alike around the world.

“Of course, I can only guess what my father would have said about the 40th anniversary,” says his son Giles, who recently published a children’s book of his own about a dog called Douglas.

“But knowing him, he would have made some quip along the lines that at least Mr Happy doesn’t have to worry about going bald at 40, while Mr Greedy’s middle-age spread is coming along nicely, and Mr Jelly would probably now go through a mid-life crisis and start going to the gym.

“But, of course, he would have been overjoyed that they had lasted so long. He would have loved the fact that now children from as far afield as Singapore and Boston and Bradford all know what a greedy or a bump or a tickle look like,” adds Giles.

His older brother Adam, whose innocent question was the spark of inspiration for the books, is now the writer and illustrator for the series.

Like many artists who go on to receive worldwide acclaim, Roger Hargreaves initially had difficulty getting his work noticed.

Ironically, the concept of the Mr Men proved too difficult for publishers in the 1970s to grasp, says Giles.

His father’s luck turned when he bumped into an old friend who published industrial pamphlets and told him about the concept for the series.

The books were printed on the off-cuts of his printing presses, resulting in their iconic square-shape.

Roger, who had a successful career in advertising, saw the potential to create a major brand out of the Mr Men, a new concept at the time.

As well as the books and TV series, the Mr Men and Little Miss characters can also be found on everything from clothing to sweets and from toys to toiletries.

Despite the runaway success of the books, which sold more than a million copies in their first three years on sale and allowed Roger to leave his job in advertising, the father-of-four was always at his desk by 9am ready to spend the day creating new stories and the Mr Men cartoon strip.

“His day would always start in the most eccentric manner,” says Giles.

“I have no idea why, but every morning he would begin his day with an invigorating icy cold bath. I assume this was left over from his Yorkshire childhood when there would have been no hot water.”

As a boy, Giles helped to open the sackloads of fan mail sent to his father and address and lick stamps for return letters and drawings.

“He would read us the original Mr Men stories at bed time, but I don’t think as a little boy it actually clicked that these were done by him,” he says.

“But my first real memory of the Mr Men was one occasion when I stole his razor blades and cut my finger. I was immensely impressed that it was patched up with a Mr Bump sticking plaster. I’d never seen an Elastoplast with a small drawing on it before.”

Giles, who lives in Thailand with his partner and two children, says he has often wondered what his father would have done if he had lived longer.

“The idea that his simple, graphic, cheerful stories have entertained millions of children all over the world would have delighted him,” he adds.

“I also think it is wonderful that the parents who loved them so much when they were young are now reading them to their children.

“Well over 100 million books have now sold, and that’s a lot of bed time stories. It’s incredible to think how many children have gone to sleep dreaming of Mr Happy. Not a bad legacy.”