Niklas Hed, co-founder of Rovio Mobile, maker of Angry Birds

The game, released in December 2009, has been downloaded more than 200 million times.

In May of last year, Angry Birds finally became the best-selling app in the United States. Slash from Guns N' Roses later tweeted, "Angry Birds is like a drug, only cheaper." I listened to their music a lot as a kid, so I thought, If he says that, now I actually believe it.

I was very positive that the game was able to do what it did, but I was nervous and felt we needed to keep this going and not lose the pace. I've become paranoid in a way. How do we keep it fresh? What is the next move? How do we stay No. 1?

I feel like we are walking, and we should be running. Every day, I'm thinking, Where are our weaknesses? One by one, we are taking steps to tackle those. And those steps, I would like to take faster. The appetite is growing as we are moving forward.

When we released Angry Birds, we started getting fan mail saying, "I love the game, but please, please, please, make more levels." So one of the first objectives was to make more levels. And they started complaining again to make more levels. We kept getting the same message. We had about 60 levels when we first released it. Now we have, like, 300.

If you like something, and it's all the same all the time, you will get bored. We try to keep coming up with fresh new content in different forms. It might be a different game, or it might be a movie. If there is an update coming out, we think about what kind of toys we should have to include with that. We have loads of ideas for other characters, but the existing ones want to be in the spotlight. Angry Birds is going to be on every platform out there.

I'm not getting comfortable. I know how fragile the gaming business is. On one day, you can be No. 1, and the second day, you're gone. There were a few times that we lost the No. 1 position, and I have to admit, I don't like that. At all.