Yo. It seems so simple. So mindless. It’s only slightly less boring than “Hey” or “Hi,” if only because of some perceived aggression or excitement attached to it. But Yo is anything but simple. If you haven’t been following along on Twitter, Yo is the hottest new app that will leave you scratching your head. The entire premise of the app is to send other users a single word: Yo. Yo currently has over 50,000 active users, after launching as a joke on April Fools’ Day. Users have sent over 4 million Yo’s to each other. Without ever having officially launched, co-founder and CEO Or Arbel managed to secure $1.2 million in funding from a list of unnamed investors, except for co-founder, angel, and Mobli CEO Moshe Hogeg, who participated in the round.