He's talking about global warming; but first he has to explain Einstein's theory of relativity and the speed of light. "I've never got round to doing this in kilometres," he says. "It's 93 million miles. I do try to drag you lot kicking and screaming into kilometres and kilograms, but for now, miles will do." They all laugh appreciatively. There are frequent references during the lesson to their age and to their shared experiences from earlier decades.
This is Geological Matters, one of no fewer than 15 University of the Third Age activities taking place today in or around Peterborough, where the U3A has 1,100 members, a number that is growing by around 100 every year. Each pays pound;13 a year to belong.
Subjects, which range from antiques to water-colours, and from Shakespeare to Scrabble, are decided by a monthly committee meeting. In Olive Potter's cross-stitch and embroidery class, taking place right now in the conservatory of the Goodbarn's Yard pub, the age range goes up to 86.
"I've always been interested in craft but I didn't have much time when my children were small," says Mrs Potter. "I worked as a clerk at the hospital, then after I retired I met a friend who had just come from a class. So I tried to get into the cross- stitch class, but it was full, so I started my own." Now Olive Potter oversees 48 students at three weekly sessions in her local pub. Learning, according to the U3A philosophy, can take myriad forms. It can be a group walk in the country, a coach load on a theatre trip, or an informal singing group in someone's front room. Since its introduction to this country from France 23 years ago, the organisation has grown rapidly. Its UK arm now consists of 500 independently run groups, with more than 140,000 members.
U3A teachers are all volunteers and classes take place in whatever accommodation local members can find; the national umbrella body, the Third Age Trust, relies on membership fees as its main source of funding to run an office in Bromley and a magazine.
Many of the members here in Peterborough are retired teachers, but few of them want to return to teaching their old subjects. Paul Parker, a former maths teacher at Bretton Woods school, organises day trips and runs a monthly music appreciation group. Jean Moyses, who retired two years ago as head of art at Crowland school in Lincolnshire, attends his "musical alphabet" and has also taken up patchwork and quilting.
"People have said, 'Why don't you take a class?'" she says. "I don't mind doing the odd workshop, but not regularly. I just want a chance to 'do' at long last."
U3A: 19 East Street, Bromley, Kent BRT1 1QH; www.u3a.org.uk