When you are Googling... (What Google doesn't want you to know about)
When you start a query in Google.com, the website creates a serial number to record this query, including your IP address and your searching keyword. If you are logged in with your Google account, the keyword will be saved with your account. So Google knows all your searching history, and it knows your history and your recent activity in real life.
This is not the end of story yet. After the searching, you browse through the results and short description, decide to click number 4th link of the result. Google creates a hidden query to send the serial number kEI="aE5ER_7xA6T2gQKJ0szoDA" and the item number cd=4 to the server so that Google knows what you are really interested in, from the link you click.
PS: Today (Feb 18) when I was reading Google News, I saw a new "Recommended for you" section:
That is what I am talking about.