We've all had it happen -- we think about buying something, maybe speak with a friend about it, and then the next time we go online -- we see ads for that very product.

So is your smartphone really spying on you? A recent survey found that 43% of Americans think the answer is yes.

"Yes and no -- that's how I answer it," says cybersecurity expert Scott Schober. He says most apps don't literally listen to our conversations. "The processing power -- you'd need a quantum computer to do that,” he says.

But if advertisers aren't listening, how do they know what you're looking for?

Experts say the answer usually lies in information we choose to give away.

This week, there was a controversy over FaceApp, which lets people see what they might look like older or younger. The app is made by a company in Russia, and it asks for access to a lot of personal data, including photos and contacts.

But experts say FaceApp isn't really that different from Facebook or any other free service -- because if an app isn't getting paid in money, chances are that it's getting paid in information.

"That data is being collected, mined and being sold, and that’s how these ad companies are getting better and better at targeting us," says Schober.

So are they able to predict what we're thinking about buying?

"Yeah, it's almost like they think it before we think it -- and it's a scary feeling," says Schober.

If you're online, there's no way to completely protect your information. But experts say that if privacy matters to you, you should limit how many free apps you download and how much you share on social media. You might also want to using a search engine that doesn’t track your search requests.