So if you're looking for the ultimate culprit behind new Gmail ads, look no further than the phone in your pocket.

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One morning recently, you and several other thousand people who use Gmail may have woken up to a message with the subject line: "Tame The Data Explosion," or something else unsolicited.

That email was not a promotion from a long-forgotten website or a chain letter from a loopy aunt.

Where once your email came in one big feed, now it's segmented under tabs labeled "Primary," "Social," "Promotions" and other categories.

Google says that it made the change because it "puts you back in control using simple, easy organization." That's true, but there's also a craftier reason behind the update.

In theory, you won't mind ads being emailed to you as long as they appear in a designated place.

Perhaps you don't mind the extra emails (you're probably in a minority) but want to stop ads from specific companies.

It's an important distinction for those worried that Google is handing out email addresses to advertisers (which it isn't doing).

That reassurance is still unlikely to stop millions from clicking the ads as if they were email."Instead of ads always appearing at the top of your inbox, they’ve been relegated to a more appropriate place in your Promotions category," Google told Venture Beat.

Specifically in this case, the amount of money Google makes each time someone clicks on an ad -- so-called average "cost per click" -- has been falling for nearly two years, in part because ads on smaller smartphone screens fetch lower prices, and people are spending more time on mobile devices.

Those little text ads that appear in every Google search and Gmail inbox are the core way Google makes money, so the decline has put pressure on Google to eke as much money out of its desktop ads as possible.

eg Nu Ke asks: "Like some people here, I use Firefox and Adblock.