The secrets of the ads that 'stalk' you

Imagine walking into a shoe shop in the high street, picking up and looking at a few pairs of shoes, before putting them down and leaving the store. Then imagine checking out a few other shops before popping into a newsagent, where you start flicking through a newspaper. As you do this, a display appears with the exact same shoes that you were interested in half an hour earlier, along with a deal attempting to lure you back to make a purchase.

That experience is pretty much analogous to...