The time: 1991

The place: Natal, South Africa

The man: Simon Singh, author of Fermat's Last Theorem

"WALKING on to the plane for South Africa was the first time I had ever been really frightened of how whites would treat me. I'd never thought of it growing up as an Indian in Somerset.

Nelson Mandela had just been released and I got involved with an organisation that was trying to help with education and development. I'd spent the last six years hidden in a lab, one of which was 500m under a mountain in Geneva, doing nothing but science - and had no idea what else I was capable of. Twenty-six is the age where you start asking what you want to do with the rest of your life. This was my time out to think.

South Africa was still a very turbulent country with the ANC fighting the Zulu party. The Group Areas Act - coloureds, Indians and whites living in different areas - was still in place. Up until that moment I had been politically naive about issues of race. Somerset is very cosy and comfortable; my family had been there since the Fifties and we'd been made very welcome. In science, race is not an issue. All that matters is whether you can do the work.

Fortunately, by the time I landed, the government made a statement that the Group Areas Act would be repealed, so I could live with my colleagues. However, I was still worried what people would think of me mixing with white people in the white township. Especially when I shared accommodation with two white women teachers. In England nobody would look twice, but in South Africa, inter-racial relationships were still taboo. Any excuse to cause trouble was taken advantage of.

Each day we would enter the Zulu homelands to teach at three of their schools. To start off with, everybody stared at me, the Zulus were convinced I was white because I was taller and lighter-skinned than the South African Indians.

The teaching was very old fashioned with everything by rote, so when I wrote on the blackboard, the pupils would just copy it down. Unfortunately the majority often did not understand elementary ideas, like how a pendulum swings, because they had missed out on chunks of education. It was hard work but I loved seeing students' eyes light up when something clicked.

While they were learning, I was beginning to learn about myself. Teaching allowed me to clear my head, and ask not only whether I wanted to continue with my esoteric and abstract branch of pure science but more importantly: am I good enough? I wasn't sure that I could make a real contribution; there are only a few pioneers who break down frontiers while everybody else fills in the gaps.

I thought: if you can't do something great, what is the point? For example, I have never been to a dinner party because I don't know what to say or how to behave - and if I'm not very good at something, I don't like doing it. More importantly, I wanted to do something that only I could do. I knew that if I stayed in physics there were lots of people who could fill my shoes. It was a real dilemma. All my life I had wanted to do fundamental science, at eight when my sister asked my ambition, I replied 'to be a nuclear physicist', so to abandon that completely was giving up a dream.

I found the solution through one of my Zulu pupils. He had no concept of basic arithmetic, although 16, and couldn't even cope with negative numbers.. I can still picture exactly where he sat in my bare classroom. I decided that, whatever happened, I would get through to him, and what's more, I would set a test and he would get 100 per cent. So while his classmates had their breaks, we would sit down together and go over everything again and again.

Finally, I set my test, the last question was asked and they'd swapped papers to mark each other's work. Holding my breath, I went through the register collecting the results and called out his name: Blessed Sibisi, and he had 10 out of 10. From that single moment I knew I could teach and, more important, had the patience.

In my book, when Wiles solves Fermat's Last Theorem, it is the high point of his life - it is ecstasy and, as a scientist, unbeatable. I also loved just understanding, communicating and breaking down the natural prejudices people have towards science.

Returning home, I decided to take my talent for teaching onto a bigger scale and apply for jobs in the media. On hearing that I had a job with the BBC; I leapt up and spiked a light bulb with my hair. I could smell my hair burning and I was covered in glass, but was thrilled at the opportunity.

Sikhs are not particularly philosophical people, we do well in business, the military and the professions - so particle physics seemed a little airy-fairy. My grandfather and father were both farmers, and although my parents respected my choices, they did not understand the point. But going into television was something they could appreciate. Making them proud was certainly a motivation.

Writing my book is, hopefully, also my way of leaving my mark. It is a beautiful synthesis of being able to teach, but I also get to be taught myself by the greatest mathematician in the world. I have a lot to thank South Africa for: I discovered not just what I wanted to do but also why I am the way I am."

Simon Singh's book, Fermat's Last Theorem, is now out in paperback at pounds 6.99