I am often asked why a guy like me (70 last birthday) writes books for children. Well, being a scientist, I like sharing my own fascination for science, and being a religious person, I like discussing ideas about God. And what I find is that it is much more fun exploring these subjects with young people rather than adults. Young people have minds that are far more inquisitive than those of adults; adults tend to get stuck in their ways and in their thinking.

But what makes me, a university physics professor, think I can write for young people? Well, I suppose in the first place it helps to have four children of my own, three step children, and (so far) 13 grandchildren. On top of that I go to schools and talk to children a lot. Most important of all, for many of my own books I have got children to read the early versions and tell me what they think of them.

When I was writing Here I Am! (now published in the USA as www.Here-I-Am), one hundred children, aged 12 to 14 read the early version. One of the questions I asked them was this; If you had been in Sam's bedroom listening to this conversation between Sam and God (or was he a hacker?) was there any question you would have liked to have asked, or was there anything else you would have liked to have contributed to the discussion?

Based on the children's replies, I rewrote the whole book so as to include the questions they told me were the ones really on their mind - rather than the ones I had originally included because I thought (often mistakenly) they were the questions I ought to be discussing. That's why so many children who have read the final book say it tackles the very questions they have been puzzling over for so long.

Another question I asked those children was: Do you think Sam was a boy or a girl - a Samuel or a Samantha? As I mention on the Home Page, my aim had been to hide Sam's sex; that way girl readers could think she was a girl, while boy readers could think that he was a boy. Imagine my disappointment when just about all 100 of them declared that Sam was a boy! I went to them and asked why they thought that. They gave me examples of what Sam said and did and reckoned a girl would not be so likely to say or do. So, in the next version, I cut out those things and put others in their place. During the next round of testing, I asked the same question. This time replies were balanced 50-50 as between boy and girl.

To end with, here are two questions for you when you have read the book:

• Who do you reckon Sam was - a boy or a girl?
• Who do you think Sam was talking to - God or someone playing a joke?