What makes an app take off? That's truly the million-dollar question. There are more than one million apps in Apple's store, and the top-grossing ones can pull in some $700,000 a day. The App Store ranking itself is a secret algorithm--believed to involve recent downloads, active users, and user reviews. "A viral app is a mixture of art and science," says Marcos Sanchez, a vice president of App Annie, which tracks app metrics.

Two apps that hit on the right formula are Candy Crush Saga and Clash of Clans. Both games are simple to understand and challenging enough to get users hooked. Though free, both games coax players into making in-app purchases or sharing the app with friends.

The graph at right charts the rise of these apps, using overall App Store rankings in the U.S. Once at the top, most apps stick around. "It becomes a self-fulfilling prophecy," says Sanchez. "More people find you because more people know about you."