People are becoming more and more aware that their activities online are being tracked.
But how does it work?
The other day my wife mentioned that she had been looking up some information on the Flying Scotsman. The next day, her Facebook feed was full of adverts about trains. She looked up some information on plants. Then she started seeing ads for garden centres.
How can Facebook know what you're interested in, even when you're not browsing Facebook?
The answer is trackers. Some of them are visible, some invisible, some are really really sneaky.
But to explain how it works, I need to run through how a web-browser displays pages to you ... which is coming tomorrow.
(I started writing this before Apple's Worldwide Developer Conference at the start of June, when they announced some measures to give you control of this tracking).
Take action:: How comfortable are you with tracking?