The top-grossing iPhone app makes a whopping $1.5 million a day

The mobile game Clash of Clans is free to download and free to play, so how does it pull in an unheard of $1.56 million in revenue per day?

The secret to its status as the top-grossing app for the iPhone lies in how it has perfected the "freemium" model.

For a mobile game, "freemium" means the initial download is free, but that certain virtual goods within the game's digital universe cost real cash. And while some games force you to pay extra money to access higher levels, Clash of Clans has implemented a more subtle strategy.

You can play the entire game without spending any money, but Clash of Clan's company, Supercell, relies on the impatience of its users to fuel the $1.56 million it earns every day.

Players in Clash of Clans collect resources that enable them to build defensive structures and troops. They can then use these troops to attack other players bases-in order to get more resources. You see? It's an endless cycle of building, protecting, and stealing. And an influx of real-world money into the equation can speed up many aspects of the game.

Why would you wait a long time for your troops to train when you can just spend a little dough to make them train faster? Of course, all those small purchases can quickly add up, and that's the point.

Candy Crush's business model works in much the same way, with users being able to pay real money to avoid waiting for more lives. But despite Candy Crush addiction being a well-known scientific fact, the game still only takes in $931,000 a day, well short of Clash of Clans $1.56 million.

One reason Clash of Clans sits on top is the gameplay. It rests in a sweet spot for a mobile game: simple enough for the casual gamer, but nuanced enough to have an entire community of hardcore gamers spring up around it. Clash of Clans' developers have helped nurture a network of social groups (clans) that now have their own websites, recruitment letters, and hierarchies.

Of course, it doesn't hurt that the characters are cute. Like Angry Birds before it, Clash of Clans has capitalized on the adorableness of cartoons characters, even when they are fighting.

Plus, it never hurts to have Liam Neeson promoting your game in a Super Bowl commercial.