Don't blink. Continue reading this without blinking at all. It's possible, but not easy. It requires significant concentration. And, eventually, you will blink.
This, doctors say, is a good comparison to the effort required by a person with Tourette's syndrome in attempting not to succumb to the tics that form the most visible part of the condition.
It affects approximately one in every 100 children. Twenty times that number experience either verbal or motor tics. To be diagnosed with the condition, a child must experience a range of tics for more than a year.
Isobel Heyman, consultant child and adolescent psychiatrist, says: "At any secondary school, there might be four or five kids with problematic Tourette's syndrome, and four or five with minor tics.
"It can be misunderstood by teachers. They think, if a child can suppress the tics some of the time, why not all the time? But you end up doing it more often if you try to suppress it."
Tourette's tics include squeals, grunts and repetitive gestures, which change in phases of several weeks or months; but the most notorious is the tendency to swear uncontrollably.
Only about 15 per cent of Tourette's children display such tics. No one fully understands why they choose swear words. Debbie, a teaching assistant from Tonbridge, in Kent, has a 13-year-old son. "You tell people he has Tourette's, and the first thing they say is "Is he going to swear?' I'd like to see it more widely understood in education circles. If you send a child with Asperger's syndrome or dyslexia to school, people know what it is. But our school hadn't heard of Tourette's."
Dr Heyman says: "If no one tells the teachers, children could be put in detention for squealing. You need to bend the rules for such children." She recommends you give them a "time-out" card, which they can use as a signal to leave a lesson. They might be able to control their behaviour for 10 or 15 minutes, and then run around the playground, releasing pent-up tics.
Alternatively, children with vocal tics might be allowed to chew gum in lessons. Those who make gestures can benefit from fiddling with worry beads. These children may also need to take exams in a separate classroom, or with a scribe.
Tics are often less problematic than the attention-deficit and obsessive-compulsive disorders allied to them. It helps if a learning-support assistant works with children to ensure they do not get distracted during lessons.
The condition can lead to depression in teenage years. "These children can get quite sad, fed-up and self-conscious," says Dr Heyman.
To avoid bullying, she recommends teachers tackle the topic in PHSE lessons, giving the sufferer a choice of whether or not to be present. And the school must clamp down heavily on classroom bullies, countering problems with clear explanations.
"A child with Tourette's isn't different to other children," says Debbie.
"You just say: 'If people can't accept you for who you are, then they're not worth knowing'."
WHAT IS TOURETTE'S?
Tourette's syndrome is a hereditary neurological condition, named after the doctor, Gilles de la Tourette, who discovered it in 1825. It affects 3 per cent of people. The chronic muscle and vocal tics tend to increase with tension or stress, and decrease with relaxation or concentration on an absorbing task.
Tics start usually when children are 6 or 7, worsen between 9 and 15, and decrease after that.
Neuropsychiatrists treat it, but there is no known cure.
'YOU SHOULD TREAT HIM LIKE EVERYONE ELSE'
Twelve-year-old Jack Clark is the only pupil in school who is allowed to swear at the teacher. Three years ago, he was diagnosed with Tourette's syndrome, but, from early childhood, he had displayed a range of physical and verbal tics symptomatic of the condition.
As a small child, he would thrust his arms in the air, often sending his dinner hurtling over the wallpaper.
"I thought he was doing it to annoy me," says his mother, Beverley Clark.
"We all thought he was just naughty."
Jack's eventual diagnosis not only provided an explaination for his tics, but also for his attention-deficit and obsessive-compulsive behaviour.
"I got upset when I got into trouble," says Jack. "I would say, I can't help it, and people said, 'Yes you can'. "
His Kent primary had little idea how to cope: he wasn't disciplined, and was allowed to ignore homework and spend lessons sitting under his desk.
So his mother moved him moved to Christchurch junior, in Ramsgate, where staff were eager to cater for his needs.
"He needs discipline," says Mrs Clark. "You've got to treat him like everyone else. He needs to learn."
The school made an exception last year, when Jack began to say the word "fuck" involuntarily, and accompanied it with obscene hand gestures.
But the school insisted his condition should not affect his education. The tic was explained to his classmates who, on the whole, reacted with approval.
"They were like 'How cool'," said Jack. "But I'm glad it's over now.
Though, to be honest, all tics are as bad as each other."
A special-needs co-ordinator works with Jack, giving him extra help where necessary. She corresponds with Mrs Clark, informing her if Jack is feeling ill or restless. He is offered a scribe during exams.
Staff help him fit in as much as possible. When a boy began calling him "Donkey boy", he was dealt with swiftly and firmly.
"My friends say, if I didn't have Tourette's, I wouldn't be me," says Jack.
"And sometimes I'm glad I have it. If I didn't, I wouldn't have the friends or the help with learning I have."