All of us have an electronic data trail. Every time we shop online, use a supermarket rewards card, or share our ZIP code or email address with a retailer, our preferences and habits are stored – and our data trail grows longer.

Of course, most of us are happy to provide what we consider harmless bits of information – like what cereal we buy, and how many pairs of shoes we order – in exchange for coupons, freebies, and discounted products. But according to The Privacy Rights Clearinghouse, the information we share with our favorite retailers actually gets sold to businesses called data mining companies. And once our information is compiled in one place, it paints a detailed picture of our interests, our health, even our religious and political beliefs. Case in point: One retailer was able to identify which female customers were pregnant by analyzing their purchases – and sent them coupons for diapers.

And our constantly-updated profile is being sold and re-sold to marketers – which is why you might see ads for oatmeal on your Facebook page the day after you use your supermarket loyalty card to buy cornflakes.

The companies that profit from data mining say they only use information that people willingly share. And, ultimately, the information saves most of us a few bucks. But some experts consider data mining a threat to personal privacy. And they recommend paying cash, not using rewards cards, and saying “No” when a retailer asks for your ZIP code.