If you signed in to Twitter recently, you likely saw a pop-up that cheerily told you the company would be changing its privacy settings and you would soon see "more relevant Tweets and ads." Beneath that information is a big button that says "Sounds Good," or smaller text that gives you the option to review your settings.

What's going on? Basic economics, as a story in today's SFGate explains. Tightly targeted ads are the best way for sites to make money selling advertising, but so far, Twitter is only getting about 1 percent of those advertising dollars (Google and Facebook get much more). Twitter's revenue was down last quarter, and both the company and its investors want to fill its coffers with tightly targeted advertising money. The new privacy policy will allow the company to provide advertisers with much more detailed information. The policy goes into effect on June 18.

Twitter knows how old my car is

The thing is, you might be very surprised to know how much detailed information Twitter is already collecting about you. That's for one very simple reason: Every time you visit a website with an embedded Twitter share or tweet button (in other words, most of the time), the company uses cookies to collect information about your browsing history. Currently it saves that information for only 10 days, but under the new policy, it will save the info for 30 days.

Ten days' worth of browsing info is already enough for Twitter to have figured out lots of things about me. For instance, I'm positive I've never posted a tweet about a car, but Twitter knows that my car is between 11 and 15 years old, that I bought it about two years ago, that I spent less than $20,000 on it, that I'm shopping for another car right now, and that I want my next car to be a compact. It correctly guessed my husband's age range, knows that I'm the breadwinner in our family, that we enjoy travel, and that I'm a "spa maven." It's provided all that information to its advertisers. How much more will it know when it starts analyzing 30 days of data instead of just 10?

In addition to saving (and sharing) more of your browsing data, Twitter also will start providing more information to its partners (i.e., advertisers) under the new policy, including personal information such as your name and email address--but only when you give your consent to the partners, Twitter explains.

You can say no

If you'd rather not have Twitter gather, store, and share all this information about you, you have the option to stop this from happening. The service gives you a lot of control over what information can be shared with advertisers, but unfortunately it defaults to sharing everything unless you opt out. To do that, go into your Twitter settings and click on "Privacy and Safety," scroll down to "Personalization and Data," and click "Edit." You'll get the option to disallow all personalization, or disallow personalization across devices, etc.

Before you leave Settings, scroll down and click on "Your Twitter Data." You'll need to enter your password. Your data will start with some basic information such as your address, gender, phone number (if you've activated one), and places you've been. But then you can click on interests from Twitter, and interests from partners--which is where this gets really interesting. Here is where I discovered how much Twitter knows about me, without my having told it a thing.

How much does it know about you?