Sixteen people - mostly male - are standing in a semicircle around a pile of joists and toilet seats. “Has any of you ever built a toilet before?" I ask.
“No. But we all had a good look at that one over there," says one volunteer, "and we're optimistic."
It is Thursday: set-up day at the Climate Camp. The previous day, the campers "swooped" and took possession of a chunk of land on Blackheath, in south-east London. By the end of the day, they had put up five tripods and several hundred metres of six-foot-high metal fencing; bussed in at least half a dozen lorryloads of equipment (marquees, hay bales, tubing and bits of wood); found the mains tap and connected up a water system;
and put nearly a thousand letters through the doors of local residents explaining why they were there and promising to tidy up after themselves.
The organisation of the Climate Camp might be the eighth wonder of the world. The whole thing is run by volunteers who meet once a month at locations around the country. There are subcommittees to deal with each aspect of the camp, such as media, process (how things are done) and the site; plans are presented at the meetings and OK'd or refined. Anyone who has been on one patch for too long is moved to another one in order to prevent de facto leaders emerging.
The toilets are a perfect example of the agonising but productive process behind each year's Climate Camp. All Thursday, every time I pass, the group is there, talking a lot more than hammering. "We're divided on the toilet seats," one of them says. "It's the great squatting debate. Hard work, you see."
“Ah yes," chips in another, "but great for your quads."
Around the camp everyone else is busy, too. They've never had so many people at this stage of the camp, so things are getting done quickly. With a huge cheer and a swoosh, the marquee goes up. On Friday the workshops start, and by the end of the weekend (so the plan goes) everyone will be ready to head straight off to chain themselves to the nearest banker.
It's a plan that has worked before (although the police are invisible this year, which, one camper says, makes it all feel a bit less radical). The ideal of "movement building" is an important one, and by Sunday campers have voted that in October they will invade Ratcliff-on-Soar power station; if half the camp turns up it should be worth seeing.
Most of the toilets are up now. "Just poo" or "Wees only", they are labelled: some toilets contain straw bales on to which you can wee, while others are built on top of bins into which you poo, then pour a scoop of sawdust.
Someone shouts from a passing car: "Have a wash." One of the group constructing the toilet yells back: "We're building a bathroom!" One of the great aims of Climate Camp is to get people to learn to organise for themselves; building toilets has got to be as good a start as any.