By Alex Kirby BBC News Online environment correspondent

Day seven of Alex Kirby's diary from the World Summit on Sustainable Development in Johannesburg.

Johannesburg is a pretty sceptical town these days, if not outright cynical.

Davis says the summit should concentrate on improving children's health

Many of us doubt that very much will emerge from the summit when all the talking is over.

Today, though, for an hour, I found myself almost hopeful again.

I had lunch with a 17-year-old British schoolgirl, Rowenna Davis, from London.

She is afraid she may be naive. I think she may be too good to be true.

Regardless, she has done more to try to make the summit work than most of us.

Initiative

What Rowenna did was to earn money - �1,800 [$2788] - to back her belief that one person can change the world.

The Rio summit was about talking, Johannesburg is about walking

Rowenna Davis

She raised the cash by tutoring for three hours a night, four nights a week.

She took packed lunches to school, and walked instead of riding buses when she had to get somewhere.

She did not go to the cinema, and for 18 months the only chocolate she has eaten is a fairly traded brand - expensive, but she disapproves of cocoa production methods.

With the money she raised, she paid to have 10,000 posters printed, all bearing messages connected with the summit's preoccupations.

Then she enlisted a team of 150 friends to act as flyposters throughout London.

"They thought it was just one of my schemes at first", she says.

"But I get respect from them now."

Making a difference

She didn't tell her mother what she was up to until the week before the posters were delivered.

Blair has good ideas but "is failing to implement them", Davis says

And her mother didn't believe what she heard. Many teenagers who might have saved almost �2,000 [$3,100] might find other uses for it than printing posters exhorting us to save the ozone layer.

I asked Rowenna why she hadn't blown the cash on a decent holiday, or something else to indulge herself.

"I'm very cynical about what can come out of the summit", she explains.

"But I don't regret my campaign for an instant. Every individual can make a difference."

She thinks the UK Prime Minister, Tony Blair, "is a good man, who probably has the right ideas", she says.

"But he's failing to implement them. He seems very behind the US, and very afraid to step away from it."

Future worries

Asked what message she would like to give the world leaders assembling in Johannesburg, she is in no doubt.

"The Rio summit was about talking", she replies, "Johannesburg is about walking. The definition of democracy is representation of the people.

"What kind of democracy is it when you can't represent the people of the future? How long can you live like this?"

But she worries about the present as well as the future, not least about the daily toll of more than 30,000 under-fives who die from hunger or easily-prevented diseases.

"What about 12 September, and 13 September, and 14 September and all the rest?", she asks.

But it is not a question. As she leaves, she is still worried that she may be naive.

There are worse things to be.