The young lad clinging to a sheer vertical wall 40 feet above the ground waits for some strength to return to his tired muscles. His next move, a long stretch to a tiny handhold high above him will demand all his skill and determination. Suddenly he realises he has paused too long. Doubt, the fatal companion of climbers, has entered his mind.
He forces it away, braces himself, and drives his body upward with all the power left in him. It is not enough. Frantic fingers scrabble just below the hold. Doubt gives way to certainty. The lad's heart lurches in his chest and he begins to topple sideways, slowly at first then with gathering speed... until the rope attached to his harness tightens and leaves him swinging gently and securely in space.
"Bad luck," his companion on the ground calls up, feeding the rope through the belay device on his belt, which lowers the climber to the floor of the Glasgow Climbing Centre, a huge hall inside an old church, whose every wall is dotted with hand and foot-holds. "Next time you'll make it."
One of the most amazing features of climbing walls, explains Neil Wightwick, the centre manager, is that they allow youngsters to do things that the world's best climbers would once have deemed impossible: "It's a matter of improved equipment and training and, of course, the rope is attached all the way up, so if you slip on a climbing wall you remain safe, which means you can push yourself."
Safety is at the core of every activity, says Mr Wightwick, whether it's a one-off taster session or the type of course - weekly sessions throughout a term - being taken today by pupils from Hutcheson's grammar school.
"Climbing is sometimes seen as a scary, extreme sport, which it can be. But not here," says Mr Wightwick. "It's safer sending your kids to a climbing centre than to the local swimming pool. No one is left on their own here until they've learned the basics and can climb safely and competently.
"We have 40 qualified instructors. The walls and equipment are designed, tested and maintained to British safety standards, and we aim to eliminate user error by instilling good practice. Expert supervision and training is the key."
But while the brain may know a climbing wall is safe, the body takes more convincing. "We've been coming to the centre since the start of term and I do a lot of wind-surfing and climbing with my parents," says fourth year pupil Ben Kennedy. "But I can't say I really like great heights - although I don't let that stop me.
"It's OK at the top, where you can relax and get lowered back to the ground. But when you're half-way up you need to concentrate on the moves you're making and not look down."
Geography teacher Brian Williamson, one of two teachers who accompany the pupils each week, says he has been climbing almost since he could walk.
"This is a tremendous way to train for rock climbing. But for a lot of people, indoor climbing is a sport in itself, with its own culture, competitions and activities."
The benefits for young people, he says, include improved self-confidence and fitness, as well as valuable lessons in trust, responsibility and teamwork. "You are very much part of a two-person team, and there's a buddy system in which each person checks his colleague and his equipment. You rely on each other."
Climbing attracts youngsters who may not be keen on games such as rugby or football, says chemistry teacher Philip Holmes. "It appeals to kids who aren't normally big PE or sports fans. Of all the senior games options - and our school offers a wide variety - indoor climbing is one of the most popular."
As young Ben takes a brief rest halfway up the wall, his fourth year team-mate Euan McCowan, explains climbing's special appeal. "I'm dyslexic and a bit dyspraxic which means I'm clumsy on the ground. My Dad was the same and he climbed, which is how I got into it. I don't find climbing difficult, it makes me feel good, and I'm sure it helps my co-ordination."
The Glasgow Centre provides climbs that range from very easy to extremely demanding, says Mr Wightwick, and welcomes people with a wide range of additional support needs. "We have kids in here with physical, behavioural and learning difficulties. We have people who are blind, people who have Down's syndrome. Climbing is not like ball games where you have to react fast. It gives you time to think. That suits a lot of people."
Age and sex present few barriers, he says. "Girls are as good as boys at climbing. Children from the age of eight can come, accompanied by an adult, and a few weeks ago we had a man in here who was 83. Climbing is for everybody."
The centre offers a range of courses from one-off sessions to progressive weekly lessons. Schools should book in advance. Children with additional support needs are welcome.
Opening hours: 11am - 10pm, Mon to Fri, 10am - 8pm weekend. Cost per instructor (six pupils) is pound;55 for a two-hour session, including hire of all equipment. Tel: 0141 427 9550; email: firstname.lastname@example.org; www.glasgowclimbingcentre.co.ukThere are 380 climbing walls around the UK, including 66 in south-east England, 27 in Wales and 25 in Scotland. For your nearest: www.ukclimbing.comdatabases