The scene: a large dome-like tent, erected in a street in Brussels. Passers-by are invited inside. A TV show is in production, they're told. There's a guy in here called Dave, and he will read your mind for you.
The victims step inside, take their seats, and are amazed: Dave seems to know so much about them. He knows one girl has butterflies tattooed on her back. He knows another person owns an orange motorcycle.
Then he gets serious. Dave starts telling people how much they spent on things such as clothes, or alcohol, the previous month. He's even able to work out someone's bank account number. How does he do it?
The big moment is when one side of the tent falls away, revealing a team of masked internet detectives and a huge video screen. Each victim's details are shown there: all the information about themselves that they've put on the internet.
The whole thing was set up by Belgian bank Febelfin, and you can read all about it (and see Dave in action) at www.febelfin.be/en/safe-internetbanking .
It might seem over-dramatic, but the message is extremely serious. These days, some people have accounts on half a dozen or more different social networks. They might not share too much information on a single one, but the aggregated data they share on all of them is very powerful.
The internet feels very personal and private, because you use it on your computer or phone, in your own home.
In reality, the internet is like a busy public square in the centre of a city, and everything you post on it is like standing up there and shouting at the top of your voice. Everyone, and anyone, can hear you.
The wise advise from the Belgian bankers is simple: be vigilant.