Len Fisher: My big bun theory

It is one of the last great scientific questions: just how do you dunk a doughnut? Julia Stuart meets Len Fisher, the academic who is pushing back the boundaries of human knowledge

It is with considerable trepidation that I climb the gloriously rickety wooden stairs that lead up to the airless office of Dr Len Fisher in Bristol University's physics department. For starters, Fisher has a degree in chemistry and pure maths, which, in my book, makes him a quantum weirdo.

But it's not just the fact that the Honorary Research Fellow has a mind that can work out percentages and understand what the foggiest the Periodic Table is all about that I find most odd. Having read his new book, How to Dunk a Doughnut, which provides scientific answers to some of life's more interesting conundrums, there's no telling what Fisher may be up to behind his closed door. He may have lashed himself to a boulder, still trying to prove to himself why a long-handled screwdriver appears to be easier to use than a short-handled one. He could be about to decapitate the next innocent who opens his door with a misfired boomerang as he investigates further the best way to throw one. But worse, much worse, would be if he were indulging in further research relating to the final chapter – the physics of sex.

It is with relief that I find Fisher fully clothed and sitting at his desk. I quickly learn, however, as we chat on the way upstairs to make coffee, that it's a narrow squeak. The 60-year-old scientist, an impossibly short, cheery fellow with a tummy that suggests he may have taken a little too much enjoyment from his research into the science of dunking doughnuts, informs me that he had planned to wear nothing but a white lab coat and a bow tie for the interview, but his agent had persuaded him otherwise.

In the staff room, Fisher fails to find a spoon and proceeds to plunge a knife into my cup. The social howler turns out to be utterly justifiable, scientifically. "If you stir tea with a spoon, unless you hold it in just the right way, it will catch a whole heap of liquid in front and keep carrying it round without stirring it in. If you use a knife, you can tilt the blade and the liquid slides past so it all stirs up," he says happily.

Back in his office, I spring him with my killer question: "So, Len, how do you dunk a doughnut?" He bounds up, ferrets around some shelves, unearths a bag of the ringed variety and tries, unsuccessfully, to force a whole one into his coffee cup. It's so not working. He yanks it out, rips a piece off and dunks it. "You just stick the doughnut in and wait until the liquid drains out," he says. Just when I'm thinking, "Is that it?", Fisher offers another of his unique scientific explanations. "See the size of the holes inside? The bigger the holes, the less coffee it's going to hold because you need very small holes to create these high pressures that support the maximum amount of liquid. So you really want a finely textured doughnut for dunking. Would you like a bite?" he says in his native Australian accent, offering me a coffee-drenched sod of fried dough.

Fisher turned his attention to doughnuts after his success with fathoming out the physics behind the dunking of biscuits four years ago. Having been approached by an advertising agency that wanted to promote National Biscuit-Dunking Week, Fisher discovered the optimum times for dunking a variety of biscuits before they collapsed. The best performer was a chocolate digestive (the chocolate layer providing a two-dimensional crack-stopper), which he discovered should be dunked at a shallow angle, so that only its bottom got wet, ensuring that it stayed intact for longer.

As well as earning him worldwide publicity, his endeavours were published in the science journal Nature and, to his great honour, won him an IgNobel Prize, a spoof on the Nobel Prize that is awarded by Harvard University for achievements "that cannot, or should not, be reproduced".

"That retention process in a biscuit is incredibly complicated," he says, "because when you put it in hot tea, all the sugar dissolves and the starch starts to swell, so that it all starts to fall to bits, whereas a doughnut is just an elastic network that holds together, which makes them really good for dunking. I started thinking about it just because I got bored, I suppose."

Other questions Fisher tackles in his book include the scientifically correct way to throw a boomerang (tilt it right back over the shoulder and release it with a whipping action, launching it at a few degrees above the horizontal to counteract the effects of gravity on the flight), and how to catch a ball (a running catch is a better technique for balls launched at angles below 45 degrees, while the "get there and wait" technique is better for high-fly balls).

Fisher has also come up with what he insists is a quick method of checking your shopping bill: add all the pounds and then add two-thirds of the number of items to the "pounds" total (ie, if you've got nine items, add six to the total of pounds). He does, however, admit that his wife's way (adding up the pounds and then rounding down all pence under 49 and rounding up the rest) may well be better. To help him in his research, his neighbours in the village of Nunney, Somerset, kindly shoved endless amounts of supermarket receipts through his letterbox. One suspects that they think of him as rather adoringly batty. One night in the village pub, Fisher asked his fellow drinkers what would happen if someone riding a bicycle threw a ball straight up in the air. Some thought the ball would land beside the rider, while the majority thought it would land somewhere behind. Fisher didn't hesitate in proving who was right and leapt onto his pushbike. "I freewheeled down the hill past the cheering crowd at the pub door and launched a small stone vertically into the air. The stone kept pace with me and, a few metres further on, landed directly on the top of my head. I thought I might make a couple of quid out of that, but I had to spend it all on beers," he says.

No doubt Fisher's students also appreciate his novel approach to science. He once taught a whole term course on the physics of sex, as an introduction to the principals of physics. It included the conservation of momentum, which says that, in the same way as a shell fired from a cannon produces a recoil that drives the cannon back, so must the forward momentum of the ejaculate be balanced by the backward momentum of the body ejecting it. Fisher calculated that if a 60-kilogram man ejects three grams of ejaculate travelling at seven kilometres per hour, he would recoil at an initial speed of 0.00035 kilometres per hour.

"I've worked it out that if you're doing it on earth, it would be like recoiling five micrometres. It's not really very noticeable, but in space, by my calculations, it would take you 24 hours for the recoil to send you to the back of the spaceship so you could shove yourself forward in time for another go," he says, with a grin.

'How to Dunk a Doughnut, the Science of Everyday Life' by Len Fisher is published by Weidenfeld & Nicolson on 22 August at £12.99