The particle of silt goes bobbing along like this," says Gordon Lobbett, performing a weird dance across the floor. "Then it hits the river and down to the bottom it goes," he continues, with an exaggerated flop. "Why?" he suddenly asks the giggling pupils, and gets the answer he wants - that the river is slow-moving.
It is a sharp reminder that this is a serious geography lesson, and the students with Gordon Lobbett, who teaches at Edinburgh's Trinity Academy, are just some of 200 or so benefiting from one of a series of conferences at Inverness High School.
The conferences started as a single school pilot scheme in Kingussie six years ago. Jim Leslie, Inverness's adviser for social studies, took up the idea on a broader scale, and based them at Inverness High, where his predecessor, Ritchie Cunningham, is rector. Now in their fourth year, the conferences are supported by Highland council, and cover three subjects - history, modern studies and geography.
Each offers Higher students from this extensive region a day-long session of free lectures and workshops from specialists with particular expertise in their subject areas.
"The conferences give the students an approach which is not only authoritative academically, but also focused on what they will need for the examinations," explains Jim Leslie.
The geography event brings together almost 200 pupils from 19 schools in the Highlands, including deputations from as far afield as Mallaig, Thurso and Wick. The lecturers are equally diverse, coming from Lenzie Academy near Glasgow, Trinity Academy in Edinburgh, Monifeith High near Dundee, and Bathgate Academy, West Lothian, along with a representative of Forest Enterprise's Ranger Service.
Ritchie Cunningham opens the session in the school's main hall, with the first of two lectures to the whole conference. His topic is the atmosphere, and his attempt to clarify and focus thinking on this problematic theme is central to the exercise, but not the whole point of the day.
"Many of the students find the subject of the Earth's atmosphere the most dull part of the course, and others are worried by the science involved," he explains. "But they need to know it, and I want to show them how to focus on the fundamentals.
"The conference is aimed directly at the examinations, but in some of the other sessions we also try to get across some of the more colourful aspects of the subject."
Following a lecture on the biosphere, by Val Vannet of Monifeith High, students split into smaller groups for workshops on a range of subjects, including urban development, river basin management, and development and health.
The conferences are open to any school in the region, and attendance is voluntary. They are broad-based rather than aimed specifically at high-flyers or under-achievers, although teachers can direct pupils to workshops in particular topics. In previous years, pupils have made individual selections, but this year participating schools have been asked to allocate youngsters to the available workshops in blocks of 10, a less cumbersome process for the new administrator, Jo Monaghan.
The geography conference is run as a joint initiative with the Scottish Association of Geography Teachers, which exported the idea to centres in Dalkeith, Aberdeen and Hamilton last autumn. But Ritchie Cunningham and Jim Leslie believe the timing of the Inverness conference gives students a real boost at a crucial point in the run-up to the exams.
The effect on pass rates is difficult to quantify. But the evaluation forms students complete show a positive response.
"They complain about the hard seats in the hall," says Ritchie Cunningham, "but most of them say the sessions simplify and help them organise their work. They have a range of needs and interests, and we cannot cover everything, but we try to ensure they get something out of the day that is going to help them with their revision."