You should never teach anything the same way twice. I can’t. I can’t use the same lesson notes year after year, even though it’s the same topic. This year’s fifth year are not the same as last year’s, so they need different teaching.”

Kay Stephenson, winner of the 1998 Salters’ Prize for the teaching of chemistry, PhD, A-level examiner and head of science at Felsted School in Essex, is explaining why she prefers teaching to her previous career in industry. “For me,” she says, “it’s not about stuffing their heads full of facts. It’s about using the kids’ inquisitiveness, so when they ask that seemingly wacky question, you can use it to help them find out something for themselves. Then they know it and can remember the facts because they understand the concepts. But you have to listen out for the question, or you’ll miss it.”

Since Dr Stephenson came to Felsted School in 1994 as head of chemistry, “part of a really good team” the number of starred As at GCSE and As at A-level has been rising steadily. sixth-form science classes are flourishing and extra-curricular science activities are making - literally - a big noise. “I love the practical side of chemistry,” says Dr Stephenson, “especially anything that makes an explosion.”

Felsted, an independent 13-18 secondary with about 400 boys and girls (300 of them boarders) has welcomed her success in the competition. Headmaster Stephen Roberts says: “We knew from the moment Kay arrived that we had an outstanding chemistry teacher, and it seemed only right and proper for her professional development that she should be entered for the Salters’ competition.”

Entrants in the Salters’ Prize have to submit a CV, references and evidence of their pupils’ exam results. They then teach a lesson, observed by three assessors. Afterwards, teachers have to explain their practice. For Kay Stephenson, this was one of the most taxing aspects. “I had been teaching a sixth-form group about dissolving substances in water. We used models of water molecules and worked in small groups with huge arguments. They didn’t write much down, but they didn’t need to because by the end they had understood it.” It was, she says, one of her best lessons and she was thrilled with her class.

Then came the pay-off. One of the Salters’ team quizzed her for 20 minutes on why she had reversed the topic plan given in the A-level course. “I stuck to my guns,” she says, even though, after 20 minutes, her interrogator revealed that he was co-author of the textbook. Finally, the judges interviewed the class and assessed their learning.

The six teachers shortlisted from the original 60 then went on to a 75-minute interview before a panel of professionals, academics and industrialists in the plush surroundings of the Salters’ Hall in the City of London. It was November last year, remembers Kay, when they rang to tell her she had won the prize. “I just said, ‘Oh bloody hell’ for about five minutes.”

Since then, life has been a whirl of prize ceremonies, conferences and involvement in such Salters’ education initiatives as the Chemistry Camps (which take 150 year 10s away on a week’s practical in university labs), the Salters’ Sharing Good Practice Project (which discusses classroom chemistry) and a conference on recruitment of quality chemistry teachers. Felsted’s chemistry department has benefited to the tune of pound;5,000, some of which has been spent on mini-cameras to take photos through microscopes to help students get the microscopic picture. Kay and her husband have not yet decided how to spend the personal prize of pound;5,000. and she still owes that science sixth-form class a meal out - or so they claim.

Meanwhile, the Chemistry Club at Felsted is going strong, with plans for a sixth-form science society next year. This year’s fifth form - “some of them are just science-mad,” she says - are eager for other ventures, an electronics club, a biology group, visits and speakers on topics such as anaesthetics and the oil industry.

This Easter saw the annual Junior Science weekend, when 11-year-olds from the East Anglia region sleep in school on Saturday night, while Kay and her colleague David Everett conduct such experiments as inflating hydrogen balloons, igniting oxygen in phosphorus and making “cannon fire” with crystals of potassium permanganate in a mixture of hydrogen peroxide and ethanol. “I always say to them, if you don’t listen carefully, you’ll miss it. And, of course, it goes off with lots of bangs. They love it.”

Kay Stephenson packs a lot into her life. She is, says Mr Roberts, available in her lab for anyone who wants a bit of help and advice. She coaches hockey and finds time to run revision courses and play golf. Above all, he adds, “She is responsive to her pupils - aware of their learning.”

“I try to give them confidence,” she says. “I’d like to believe they don’t feel stupid if they ask me a question. We work through problems together.” One example, she remembers, came in a third-year lesson on compounds, mixtures and elements. “I wanted a quick half-hour lesson, but one girl looked at the compound, then looked at me and said, ‘Why has it shrunk?’ I said, ‘Why do you think?’ Someone else said, ‘Is it some kind of bond?’ - and we were off. Now, two years later, they know about bonding and energy changes, because they found it out for themselves.

“I may have ‘wasted’ that lesson on mixtures and elements, but I’ve saved four or five lessons over the years because of the students’ understanding.”

Stephen Roberts says Dr Stephenson has the courage and flexibility to take a lesson where it needs to go. But too many children still think science is boring. Why? Are courage and flexibility in short supply among science teachers? Is it, as Kay Stephenson suspects, that facts become an end in themselves? That they crowd out children’s curiosity? Or that teachers lack the confidence to follow the spark of the moment?

As full of fizz and fire as her favourite experiments, Dr Stephenson likes her job - so enjoys herself. She says:“It’s the ‘Aha! factor’ - when a child’s question makes you question your own understanding. That’s what’s great about teaching.”

More details about the Salters’ competition from Salters’ Institute of Industrial Chemistry, Salters’ Hall, Fore Street, London EC2 5DE.