"When your friends won't tell you the truth, the Ugly Meter will." So goes the tagline for the latest iPhone app sensation, which recently shot up to second place in the iPhone app store, just behind Angry Birds. The app was also the top seller in China for weeks. Created by Jo Overline and Ryan Allen of Arizona, the app has been around since 2010, but only hit the mainstream after its latest iteration, the Ugly Meter Pro, was featured on Howard Stern's radio show. Here, a guide to the iPhone's newest hit:

How does the Ugly Meter work?
The app snaps your picture, then "scans" your face, calculating its contours, symmetry, and proportions. The app pronounces your ugliness on a scale of 1 to 10 — with 10 being the ugliest — accompanied by corresponding quips. A low score will earn you praise ("You're so hot that you make the sun jealous"), while a high score will result in insults ("You're so ugly, you could make a glass eye cry").

Why is it so popular?
"Everyone is interested in vanity," theorizes Overline, "and they like to know where they rank." The app raked in $80,000 the day after it was featured on Howard Stern, and has already racked up $500,000 in revenue.

How much does it cost?
The Ugly Meter Pro will set you back $5. The original Ugly Meter is priced at 99 cents.

Does it really work?
Perhaps not. A recent test resulted in British Prime Minister David Cameron scoring a 7 ("If ugliness were bricks, you'd be the Great Wall of China"), but super-hunk Brad Pitt fared worse, earning a blush-worthy 8 ("You could walk through a haunted house and come out with a paycheck"). 

Has there been a backlash?
Yes. Parents are concerned that the app, with its focus on looks, could result in new forms of cyber-bullying and hurt children's self-esteem. Overline insists that the Ugly Meter is meant to be used in the spirit of good fun.

Sources: CBCNewsCNet, Daily Mail, SanDiego6WCNC