The danger of posting personal information online is being taught to children as part of a pilot project to keep them safe from criminals.
Pupils at a primary school in Shropshire and a high school in Cambridge have been taking part in the trial launched by the Information Commissioner's Office.
The Information Commissioner, Christopher Graham, said: "I think the danger is of jigsaw identification. You can put little bits of information out there and somebody can piece it all together.
"So the name of the game with the primary school kids is to explain to them how the online world works and how there are organisations who make money by getting a hold of your information and selling it on to someone else."
Between January and October 2012, 35 million pieces of personal information were illegally traded online - up from 19 million during the whole of 2011.
As much as 90% of that trade was in the form of login and password details.
It helps to explain why 24% of UK residents have been the victim of identity fraud.
And the need to teach children early in life became even more evident after an expert at the London School of Economics recently estimated a million nine to 12-year-olds already use Facebook, despite the fact they are meant to be 13 before becoming members.
Mark Neighbour has been leading the pilot project at St Bede's School in Cambridge with year nine pupils.
He said: "We want them to think about what type of information could be used by a third party ... Why do they need to know their age for instance? Why do they need to know where they live? Why do they need to know where they shop?
"What we want them to do is question themselves before they put pen to paper or finger to a keyboard because once the information is gone, once they hit return, it's out there and anyone can get to it and use it."
Many children are also unaware that what may be quickly posted on the internet can potentially still be seen by universities or employers checking up on candidates years later.
Melissa, 14, was one of the pupils who attended the lessons.
"The internet is really scary and you should think twice before you start posting things. What you tweet, talk about on Facebook, pictures, things like that ... it's out there forever."
Rebecca, who is 13, thinks all pupils would benefit from the classes.
"It's actually helped everyone understand what we need to be aware of using the internet and what people can do."
Schools already warn children about online safety as part of the IT curriculum but these lessons are said to go into more detail about the importance of protecting personal data on the internet and in everyday life.
If the pilot proves a success, it will be rolled out in schools nationwide.