You can see how it could happen. Most phones have cameras, so it's the work of a moment to take a picture and press 'send'. And young teenagers aren't brilliant at thinking through the consequences of their actions. One minute it's a private photo. The next, it's public.
Imagine this. Your teenage daughter sends a nude picture of herself on her phone to her boyfriend. (It might be a joke. Or it might be because she loves him. Or maybe he asked her to.) He sends it on to his friends. Now it's on everyone's phone. It's on a social networking site. She is mortified. She doesn't want to go to school any more. She says she can never go out in public again.
But this isn't some imaginary nightmare. There's growing anxiety that more and more teenagers are sending sexually explicit photographs of themselves to each other via phones and social networking sites.
In July this year, the Daily Mail carried the story of 13-year-old Sophie who sent a topless picture to her boyfriend.
"It's just normal," she said. "If a boy likes a girl, the first thing he does is ask for your picture. Some of my friends had done exactly the same thing for boys. It's just that theirs didn't get sent around.
"As soon as I arrived at school, I knew most people had seen it because they were staring and laughing. When my back was turned, I heard the words slag and slut. I tried to ignore it."
Adults generally seem to believe that online danger comes from strangers – paedophiles who groom children for sex. But bullying and manipulation can also come from the young people's own social networks
But the idea that teenagers – and even children as young as 11 and 12 – are doing this horrifies most parents.
You don't want your teenager exposing herself to anyone and everyone. You don't want her picture to end up on a site that could be accessed by paedophiles and perverts.
But how widespread is it? Has 'sexting' really become commonplace?
It's definitely on the increase, simply because 15 years ago phones didn't have cameras, and now they do.
In a speech in April, MP Ann Coffey was concerned enough to ask the mobile phone industry to do more to educate young people about the potential dangers of sexting.
But the surveys that have been done so far – and there are some that say that up to 4020Fact204_29_11.pdf" target="_blank">criticism.
Many studies have included 18 and 19-year-olds, who are teenagers but also adults. Others have based their results on teenagers who have volunteered to fill in an online survey – and teenagers who like filling in surveys may not give you an accurate picture of what most young people are actually doing.
The definition of 'sexting' also changes from survey to survey. If you include, for example, the sending of 'nude or semi-nude' pictures, that might mean a teenage girl sending a picture of herself in a bikini, which is hard to find shocking.
And, finally, if you ask a group of teenagers whether any of their friends have shared intimate photos, they might all say yes, but they might all be thinking of the same person. (A friend of mine says there's someone in her daughter's school who keeps asking all the girls for nude photos. They tell him to get lost.)
But while we can't be certain of exact figures, a recent threat assessment report from the Child Exploitation and Online Protection Centre (the UK's leading law enforcement agency for child exploitation and abuse) says, "CEOP has seen a marked increase in the number of reports where young teenagers appear to have taken still or video indecent imagery of themselves which is then shared online."
Nowadays, it seems that the vast majority of these images, according to this CEOP report, are being freely produced and uploaded 'without external influence, coercion or threat from adults or others'.
But CEOP is also aware that some teenage sexting is not so innocent.
Chief Executive Peter Davies says, "We know that young people are increasingly using technology not only to stay in touch, but to explore their sexuality...As these images can be shared so widely and quickly online, young people may become the victims of bullying or harassment. In some rare instances, these images end up in the collections of child sexual offenders.
"If a teenager were to have in his/her possession an indecent image of another minor, they would technically be in possession of an indecent image of a child, which is an offence under the Sexual Offences Act 2003. [But] As this kind of activity between children may be simply indicative of their sexual development and/or experimentation, every case is dealt with on its own merit by the CPS [Crown Prosecution Service].
"'In saying this, research tells us that many child sex offenders will be aware of their sexual interest in children from an early age (many say before the age of 18) so it is important that every case is investigated, individually judged and assessed."
For parents, the other worry about teenage sexting is that it seems to put young people into the weird sexual straitjackets we hoped had disappeared way back in the 1970s.
A report for the NSPCC published in May this year, based on interviews with young people in years 8 and 10 in two inner London secondary schools, makes genuinely shocking reading.
In these schools, girls were being hassled and harassed all the time to send the boys explicit photos of themselves.
Boys seemed to think they 'owned' their girlfriends' bodies. A girl might be asked to photograph a boy's name written on her breast with black marker pen.
The report says, "We found considerable evidence of an age-old double standard, where sexually active boys are admired and 'rated', while sexually active girls are denigrated, shamed and despised as 'sluts'."
The researchers found there was a culture of silence around all this. The year 8s particularly didn't get support when they were upset or confused by the pressures they were facing because parents and teachers assumed they were too young to be involved.
So if, as a parent, you want to protect your child or teenager, what can you do?
There's advice, help and support from CEOP which has police officers working side by side with child protection experts.
CEOP have also produced a 10 minute film called Exposed (see below) which is well worth watching with your teenager – it's the story of how a teenage girl reacts when she realises private photos she sent to her boyfriend have been uploaded on to a public site and have had thousands of hits.
You and your child/teenager can also get help from organisations like Beat Bullying which runs the CyberMentors site. This provides young people aged 11 – 17 with a safe place to access real-time, online support from trained mentors, who are young people their own age they can talk to.
But probably the most important thing you can do is talk to your child or teenager about the possible dangers of uploading what CEOP calls 'self-generated indecent imagery'.
It may start as a laugh, or a bit of flirtation, or a way to break up the boredom of a Sunday afternoon. But once out there, the photo can't be taken back. It can end up all round the school. It can end up on a public site where your mum, your dad, your granny, the man in the corner shop and the weird guy in the park can see it. It can be circulated among the kind of people you would never, ever in a million years want to see you naked.
Top 10 tips for parents
1. Talk to your child or teenager about their digital footprint. Whatever you upload on to a phone or an internet site is no longer private. And it stays out there forever.
2. Talk about sexual bullying. If someone asks you for a nude photo, it might be because they like you or fancy you. But it might be because they want to humiliate or blackmail you.
3. Discuss how hard it is not to do what everyone else is doing, whether it's getting drunk, taking drugs or circulating nude pictures of someone you know. But you have to take your own decisions. You have to have self-respect.
4. If your child or teenager is upset about a picture of themselves that's already been circulated, have a look at the advice on ChildLine.
5. After discussing this with your child/young teenager, talk to their mobile phone operator about filtering software to block inappropriate content and websites...
6. ...and ask their mobile phone company what they're doing about educating children and teenagers about the risks of sexting.
7. Don't be intimidated by technology. Find out more at www.thinkuknow.co.uk.
8. Don't assume young people are able to look after themselves by their early teens. There are always new challenges. Keep up-to-date with what's going on...
9. ...but don't check their phones and computers. It tends to close down communication – and young people who want to protect their privacy will develop multiple profiles.
10. Keep talking about online safety. Encourage your child or teenager to talk about any kind of behaviour that worries them with an adult they trust – you, a teacher, or another parent.