Do you use Gmail? Chrome? Youtube? Google? Google +?
Oh and do you use Gmail on your phone as well? Do you have an Android phone, do you download apps, free or otherwise?
Do you sign in with your Google accounts? Have you OK'd Googles new cookie policy?
If you've answered yes to only one or two of the above, it will seem like Google can read your mind, if you answered yes to ALL of the above, they practically are reading your mind.
Here's a little topograhical, schematic.
Turn on computer > start up chrome > Login to Google > Search lawnmowers and garden gnomes > Click on first sponsored link > accept cookie policy > browse site > make purchase > Go onto Google + and browse interests > click through to forum and reply to post > Leave house, take the train, turn on phone, login to Facebook post about lawnmower purchase.
Now in that above scenario, you have entered into Google's world and you have agreed that they can mine certain data about you. So when you click onto the sponsored link and accept cookies, Google is tracking your every move on that site. Then after you leave and email your friend, Google monitors keywords through your account.
Go on to Facebook with your phone? Well guess what if you have an Android, that operating system was made by Google; so it knows every site you go to, the keywords of every post you do and it analyses and sends back to the algorithm, which will come up with an appropriate action.
Stuff you post on forums from your computer is also being tracked, especially if you're signed into Chrome as you browse, keywords are collated and analysed and the deep learning, fuzzy logic, algorithm, prepares adverts and offers for you.
Ultimately if it bothers you, don't sign into Chrome when you are browsing, always sign out of your Gmail before browsing, don't access Facebook and other forums from your phone (good advice anyway).
Don't post anything on the web you're not happy for Google to know the details of and after a short while, they will seem less prescient and like they can read your mind...Woooooo!!! :)