Guess what? If you took all the neurons in your brain and laid them end to end, they would go round the earth 25 times. I know this because a man told me. As that man is Professor of Public Understanding of Science at Oxford University, I believe him. And yes, I am slightly amazed.

This, reckons Richard Dawkins, is just the kind of thing to catch the imaginations of school-age scientists. Big numbers. Facts their parents don’t know. Extraordinary feats of natural engineering. Counter-intuitive calculations. (Did you know that every time you glug a glass of water, you swallow at least one molecule that passed through Oliver Cromwell’s bladder? Neither did I.) If this all sounds a bit gee-whiz for the author of beautifully argued best-sellers such as The Selfish Gene and The Blind Watchmaker, it is not because Dawkins is about to start a new sideline as a children’s author. These ideas stem from his advice to Oxford University Press as consultant editor for their hefty new The Young Oxford Encyclopedia of Science.

This is new territory for the great explainer of evolution, whose life’s work has been trying to convey the detailed implications of Darwin’s theory to people less gripped by its profundity than he is. Five of his six books to date have been about Darwinism, and they have brought him fame, fortune, and a reputation for sharp responses to his critics.

Evolution, and the ways in which it makes the world safe for a rather strident atheism, still seems to be his main interest. His last book, Unweaving the Rainbow, was a broader defence of science against the charge that it drains the world of significance, but he is wary of general pronouncements. It would be easy to become Professor Soundbite. True, his occasional forays into the comment pages of the newspapers - most recently arguing that it was the religious delusion of an afterlife that paved the way for the suicide attacks on the US - can be intemperate.

Yet in person he can be almost reticent. And, in spite of his job title, he seems pretty disengaged from the broader discussion about public understanding of science. He frankly admits that he knows little of what is done in schools. In fact, his privately endowed chair carries few obligations beyond writing. As The New Yorker put it a few years ago, he is the Oxford Professor of Being Richard Dawkins.

Sitting in the autumn sunshine outside the handsome house in Oxford where he lives and works, he is still thoughtfully cautious about what he knows how to do best - the business of explaining science to other people. Modest, too. It helps not to be too quick on the uptake, he says, to have had to work hard at it yourself. “People who are effortless understanders are sometimes not very good at explaining.”

It also helps to spend a few decades as an Oxford tutor. All those individual conversations with students gave him an enormous amount of practice at noticing the things that people find difficult.

“I think that the Oxford tutorial system, which I hugely enjoyed as a victim, so to speak - as an undergraduate - is also an extremely good training for a teacher, and for a writer who strives to be clear. You listen to thousands of questions from students, and so have learned what are the problems. You think ‘Oh! That’s what you don’t understand. OK, that means I’ve got to twist it round and tell the story in a different way’.” Modesty returns:

“I’ve never tried to explain any really hard science, like quantum theory”.

A writer of books also has advantages that a teacher checking off national curriculum targets, or the author of an encyclopedia article for that matter, may envy. Books are long enough to cope with the ways in which one scientific explanation tends to be embedded in another.

“I do sometimes have the sensation that you start off writing something thinking it’s going to be all over rather quickly, and then it kind of grows. It doesn’t grow at the end. It grows in middle as each bit needs explaining in its own right. So you sort of embed a new chapter or sub-chapter in the middle. And if you have the luxury of being able to do that - if you are writing a book as opposed to a bit of journalism - you really can roll your sleeves up, step backwards and think of the difficulties that the reader might have. And approach it from a new direction.”

But that only works if you’ve got the reader hooked. So how to get them interested in the first place? For anyone, children or adults, Dawkins reckons there are two ways to make science appealing - “call them the non-stick frying-pan view of science or the Hubble space-telescope view”. The promise of neat applications, in other words, or the path to wonders.

Then there is a third way, which he is frankly dubious about, “what I call the exploding custard school of thought”: science as Fun. As he says, this is the approach which tends to dominate “these Science Week-type events”.

The objection needs elaborating, lest he be quoted as “the Professor who is Against Fun”. It’s fine in its placeI “but where you try to tell people - children especially - ‘science is fun. Science is bangs and smells and exciting things happening’, I’ve always thought that that’s selling science too easily and giving a false impression that if you do science you’ll have nothing but fun.”

This is not as austere as it sounds, although there is a touch of austerity about Dawkins, even when he is rhapsodising about the wonders of the world. For him, the real fun comes from the understanding which follows the hard graft.

And there is so much to understand. “The children who are alive today are tremendously lucky because we do know so much, and so they are in a position to learn not quite everything about the world in which they find themselves, but a truly enormous amount compared to previous generations.”

And telling them amazing facts and figures is a good start. But the best ones are those with a theoretical point to them. As a biologist, he thinks taxonomy - the classification of species and their relationships - is particularly appealing: “Did you know that a tuna is a closer cousin to you than to a shark? I remember as a child when I first learned that sort of fact - I think from my father - being intrigued by it.”

Then there are facts you can approach more craftily. Take echolocation in bats. You could simply say that is how bats navigate. But better, Dawkins suggests, to pose a problem. “Bats have to find their way round in the dark. How do they do it? Then make a list of things which might work, saving the disclosure of the right one for a closing flourish. This is a very helpful teaching aid, partly because it becomes very hard to forget. When you want to tell the story again, the facts stream out in a sensible order which is given you by this problem: possible solutions: actual solutions format.”

The caution is gone now, and his enthusiasm shines through. Anyone letting him loose on a class of schoolchildren would probably want to keep him off the subject of religion - but if you want to get kids interested in science, an encounter with Richard Dawkins would probably work pretty well.

Jon Turney is head of the department of science and technology studies at University College London. The Young Oxford Encyclopedia of Science, pound;30. Linked website: www.oup.comscience-encyclopedia