Free app Yo does just one thing, and nothing else. Its maker, Life Before Us, touts it as "the simplest and most efficient communication tool in the world," and "a zero character communication tool." But how simple is too simple?

Yo makes it ultra-easy to reach out to friends and family. Download the app (which takes all of about three seconds) and open it. After going through a quick phone number verification, you're asked to let Yo find your friends that have the app. When it's located your friends, it lists them in big, color stripes. Tap one of their names to "Yo" them. It's basically a way of saying "hi," "I'm thinking about you," "What's up?", and so on. Facebook users might define this as a close sibling of the poke.

When you tap on a name to send a Yo, the app says an audible "Yo" in a voice that sounds a bit like WALL-E's mechanical speech. Your friends get a "Yo" notification using the same voice as its notification sound. That's all there is to it. Everything about the app is as simplistic as it gets. Even Yo's icon is stripped down in the extreme; it's just a blank, purple icon.

Yo's meteoric rise took a dramatic turn a few days after its launch when TechCrunch learned that a group of Georgia Tech students had quickly hacked the app. The students claim to have access to every Yo user's phone number, and that they're able to spoof, spam, or send push notifications to any user. They also say it's easy to change out the app's "yo" message with any text they want. The company behind Yo has reportedly "brought in a specialist security team" to fix these problems.

Other problems have started popping up, too. One user signed up for the username "ELONMUSK" and fooled other Yo users into thinking he was the real deal. Not every issue is bad, though. Yo's sale page on the iOS App Store has dozens of hysterical user reviews extolling the virtues of the "life-changing" app.

Yo is the brainchild of Israeli programmer Or Arbel, who built the app in a mere eight hours. Remarkably, Arbel managed to raise $1 million in funding from a collection of high-profile investors. Yo attracted hundreds of thousands of users in a just a few days' time, shooting it up the App Store charts.

It's a free download for both iPhone and Android.

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