There is a loud bang and everyone gasps - a foaming green potion rises over the top of a giant test tube and strange purple liquid is bubbling away in a flask over a flame. An old plastic dry-cleaning bag drifts up to the ceiling like a hot-air balloon - there's never a dull moment in Grandad Goodwin's science lab.
The children of Keithhall School, in Inverurie, are mesmerised and three small boys at the front are bursting with pride. That's their grandad up there in the white coat - mixing strange liquids, setting things on fire and generally looking like he's having very good fun.
His excitement is infectious and he's quite a showman. He insists he's not a magician. "It's not conjuring tricks," he tells the children. "This is science."
Some of the children are only six or seven and the concepts they're hearing about for the first time may feature in their finals in 2025. But today Grandad Goodwin is giving them a head start.
He's well placed to introduce them to the world of science, as a former science teacher whose passion for chemistry started when he was not much older than them. Alan Goodwin taught trainee teachers how to teach science and retired as head of the department of science education at Manchester Metropolitan University. He's now 72 and, in his role as visiting research fellow in chemistry and materials, delivers demonstrations like this on a regular basis to children and teachers.
His three grandsons go to this Aberdeenshire school in an idyllic setting next to a country church, a few miles from Inverurie. When the boys' headteacher, Kate Lamb, heard about their grandad, she jumped at the opportunity - today is the second session he's delivered and the focus is on "materials".
"The first time he came, it was like watching a Royal Institute Children's Christmas Lecture on the TV," says Mrs Lamb. "So we had bangs, we had explosions - we had all sorts going on. The kids were fascinated. The best thing was it was one of those situations that could really inspire them to become scientists.
"My class last term did materials as a topic, so they've already looked at solids, liquids and gases, melting and freezing and soluble and insoluble gases. Alan had a list of topics, but we had to find something that was suitable for the whole school," Mrs Lamb explains.
There are 46 on the school roll and Alan is pitching his demonstration at a level to hold the attention of five-year-olds without alienating the 12- year-olds. His grandsons - Alex, five, and twins Matthew and Douglas, eight - are understandably excited today and it's a bit of a family occasion. Mum and Dad have come in to watch and their grandmother Pam, a retired primary school teacher, is assisting Grandad with his lab equipment.
The boys' father, Stephen, works for BP and is one of the educational link co-ordinators on the oil company's Schools Link programme. Their role is to promote STEM (science, technology, engineering and maths) subjects and they often establish links in their children's schools. Stephen's involvement here prompted him to offer his father Alan's expertise.
"Dad used to bring experiments home and open them up on the kitchen table and show us what things did. He's a quintessential scientist," says Stephen, now a chemical engineer inspired by his father's passion.
`ALMOST A DYING ART'
The magic of chemistry has worked and there are rave reviews from the pupils of Keithhall School - even the ones Alan Goodwin is not related to.
"Awesome," says 10-year-old Robert Laverty. His favourite moment was the big explosion. "When I'm older I want to learn chemistry to be an animal scientist," he says.
CSI: Crime Scene Investigation fan Lucy Ellington, 11, would rather be a forensic scientist. "I thought it was really, really fun," she says.
Mr Goodwin's grandson Douglas Goodwin, eight, says: "I really liked all of it, especially the elephant's toothpaste."
The green foaming toothpaste, his grandfather explains, was the result of decomposing hydrogen peroxide and making oxygen. "Because I had got detergent in there, the bubbles of oxygen were caught in it to make that foam," he says.
As the flasks and glass containers are packed away until the next time, Mr Goodwin chats about life since "retirement".
"I do quite a lot of this for teachers," he says, "because it's almost a dying art, partly because it became unfashionable to do a lot of demonstrations and then, I'm afraid, it became too bureaucratic with health and safety."
He worries about the way children learn science nowadays. "One of my beefs about the way science tends to be taught now is that it's so important that schools get good exam results that they tend to teach science to get marks, not to learn science. You just need to know that when that question comes up that's the answer.
"If you work at something so that you understand it, you actually don't have to learn it - so it takes away the strain on the memory."
Details of demonstrations at www.scitutors.org.ukarticle.php?id=114.