Carolyn Fry visits an alternative energy centre
Six students stand on a map of the world, giggling and clinging to each other. Together they represent the world's six billion people.
They're allowed to stand only on the land and if they put a foot in the sea, they drown and they're out.
With careful balancing they just about manage it, showing that our planet can support its population if its resources are equally shared. But when one girl is given huge cardboard shoes to wear, the story changes. Her big feet, representing the energy-guzzling developed world, take up too much room and the other children fall into the sea.
"That's what happens when one group of people try to use too much of the world's resources," explains Anne MacGarry. "There's not enough to go round and some people suffer as a result."
Anne is an education officer at the Centre for Alternative Technology in Machynlleth, Wales. She's teaching a group of 24 GCSE geography students from the nearby secondary Ysgol Bro Ddyfi.
The 14 and 15-year-olds have come here for the day to learn about "ecological footprinting", which is a way of calculating the impact different activities have on the planet.
After a video about energy use and climate, Anne splits the class into groups and hands each a food product. One table has an organic Welsh potato, another a pack of microwaveable oven chips, another a pack of organic Welsh crisps, and so on.
The children have to place picture cards on the table to represent the uses of energy involved during their product's manufacture. The microwaveable chips, presented in plastic and cardboard packaging turn out to be the undisputed environmental baddies.
Finding "lower impact" ways of living has been the Centre for Alternative Technology's aim since it started in 1974. The 40-acre site has displays of wind, water and solar power, and 11 members of staff live there as a working demonstration of alternative technology.
The centre aims to practise what it preaches. The building in which we are sitting has a wooden frame and walls made of straw bales. It is rendered with lime clay on the outside to deter the worst Welsh weather, while we're kept snug inside by a low-energy woodchip burner. Across the way, the award-winning information centre and shop is built of compressed earth with an insulating layer of sheep's wool.
"When the building reaches the end of its life the glass windows can be removed and recycled and the rest will simply decompose," says media officer Amanda Roll Pickering.
After a morning in class, the students are keen to let off some of their own energy exploring the centre's outdoor exhibits. The wave machine and wind seat are instant hits. The centre offers everything from day visits to residential stays at all levels between key stage 2 and A-level. Topics range from food chains and consumerism, to climate change and sustainable development.
During sleep-overs, energy for each cabin's hot water and lighting comes from roof-mounted solar panels, along with wind and water turbines. Each cabin has an "electricity board" where the inhabitants can monitor the amount of energy they use. "If they use too much, the lights go out, or there's no hot water," says education officer Verity Jones.
Today's group is only here for the day and the final exercise is to list ways in which they could reduce their ecological footprint.
"The exercise with the food products was important," says 15-year-old David Cornell.
"I didn't think before about how just one product uses lots of machinery.
I'll think more now about how much it damages the Earth."