Chances are that if you know somebody with a Nintendo 3DS right now, you know somebody who's playing Animal Crossing: New Leaf. The game was already a smash hit in its home territory, where since its release in November, it's sold 4 million copies. Nintendo revealed via its Facebook page that five days after it hit the U.S. market, it had already shifted 200,000 units.

What makes the game so enduringly popular? Months after its initial release, you can find Japanese players still Tweeting fresh photos of their towns and their homes. How do you design for longevity? And why is the game gentler than its predecessors?

At last week's E3, Gamasutra sat down with Nintendo producer Katsuya Eguchi -- also the producer of the Wii Sports series -- to ask him some questions about the game's design.

It's a world with rules based on the way things work in the real world, in terms of economy and human relationships. How do you decide how those kinds of rules work in a game like this?

Katsuya Eguchi: We take these into consideration when making all games, but specifically with Animal Crossing, our intent was to create kind of a parallel world, a world that's kind of similar to your own but also different.

And you spoke about relationships. There are people in your real life that you like and some that you don't like. There are different personalities that you are drawn to and others that you might want to stay away from. Also, people look different, so all of the characters in the game are created to look different.

You mentioned the economy. We created stuff that you might want to buy, but in order to buy those things you have to make money, and in order to make money you have to work... our intent in creating in Animal Crossing was to create this mini-world that you live in, so certainly we take into consideration those rules that you have to live by in the real world, and those are reflected in the game as well.

By necessarily creating a world and creating rules, you also create a set of values. By what's valued in the game, what you value as a person is communicated through the game. Can you talk about that?

KE: You speak about values; everyone has different values. Some people might think a certain item has more value than another does, or may think it's worth it to spend bells on a particular item when someone else doesn't. It's all kind of like things in this life -- this value system.

Your town becomes a reflection of your values. The time you spend on relationships reflects how much you value that relationship. As someone creates their town it's a reflection of their values, and when you show it to people, it should be something that you want to be show off to people. It's something that you should be proud of.