If mobile gaming is a multibillion-dollar industry, then how are developers making money from free apps? And if so many apps are free, why would users pay for apps at all? The answer lies in the "freemium" business model for apps, in which apps offer some services for free, incentivizing users to try an app before paying for additional features. The primary source of income for many freemium apps is in-app downloads, and a recent study from Flurry found an increasing number of freemium apps populating the top 100 grossing games list in Apple's (NASDAQ:AAPL) App Store. Special feature