This was visibly demonstrated to me this morning. I was reading an online forum about how a politician may be suffering from Vitiligo. I didn't know what it was. I had to Google it. It turns out that Vitiligo is a condition where you lose pigmentation cells in your skin, and you get big white blotches that will never tan again.
The next thing that I did, was close the tab and navigate to another blog. And what do you know -- the advertisement next to the blog was showing me a treatment for Vitiligo. Coincidence -- I think not.
Big Brother reads my browser history to try to sell me stuff. It's a good thing that I wasn't googling pictures of hairy French babes.