Google's free Arts & Culture app has rocketed to the top of the Play Store and Apple's App Store. But it's not due to an unexplained burst of interest in cultural works; it's all thanks to a new feature that matches users' faces with lookalikes from a catalog of historical paintings.

You've probably seen examples of the feature's ability to identify artistic doppelgangers on social media sites like Twitter. Once someone uploads a photo, the AI resizes and reorientates it to get a better view. It then identifies unique "fingerprint" characteristics such as the distance between the eyes and the size and shape of the nose.

A person's features are compared to over 70,000 images in the Google Art Project database, and the portrait that most resembles the selfie is surfaced. The result also includes a percent figure indicating how closely the two images match.

Talking about the app's explosion in popularity, Patrick Lenihan, a spokesman for Google, said: "It took us by surprise. I'm the PR person and I didn't even pitch anybody."

The update arrived a few weeks ago but has only recently started to become a viral sensation, much like Microsoft's Guess Your Age and Guess Your Emotion facial recognition tools from a couple of years ago.

As is usually the case with image recognition apps, results do vary, with some portraits resembling the selfies a lot more than others. Fall Out Boy's Pete Wentz has been sharing his results on Twitter, as have Felicia Day and a slew of other celebrities.

Google has emphasized that it doesn't keep the photos once the comparisons have been made and that it doesn't use the images for any other purposes, though some people, including actress Alyssa Milano, are skeptical of these claims.

Right now, the feature is only available in "select locations." It's not officially available in Illinois and Texas, likely due to the states' strict laws on the use of biometrics such as facial scans.