For many weeks after Apple's App Store opened its virtual doors, the most popular application in the place turned out to be nothing more than koi swimming lazily in a pond. And yet, if you were to glance at the Top Paid Apps listing at the App Store any time in recent weeks, you'd usually see Koi Pond holding down the top spot. Even now, with heavy hitters like Spore Origins and Star Wars: The Force Unleashed to contend with, Koi Pond remains a mainstay on the App Store's popularity charts.

Just how many downloads does it take to dominate the iPhone bestselling paid app spot for four weeks running? The Blimp Pilots, the five-person team behind App Store phenom Koi Pond, isn't telling. But there is a lot to learn from Koi Pond's journey from idea to actuality, and designer Bill Trost and engineer Brandon Bogle were more than happy to share with Macworld some helpful advice for aspiring iPhone developers.

What prompted you to make the leap from designing entire gaming worlds like Everquest to working on apps for the iPhone in your spare time?

Trost: When Apple announced their developer plan for [iPhone] 2.0, we just thought it seemed like a great idea. None of us had any real Mac experience before, but we thought we'd give it a shot. So we went out and bought Macs just to take part in iPhone development. We did it as a hobby, mostly for fun, thinking that after developing a few apps maybe we'd be able to pay for our computers.

Was making the jump from game development to Objective C and the Cocoa application environment difficult?

Bogle: It was challenging at first, of course. None of us had any prior experience with modern Mac development. I had done some professional game development back in the System 7 days, but none of that helped now. Objective-C was not very difficult to learn however, and being a strict superset of C, we were already very comfortable with the core language.

I was pretty impressed with the Cocoa framework. You get so much functionality, and consistency in look-and-feel, with surprisingly little work. Interface Builder is a very powerful tool, and it really helped me understand how things all fit together. There was a point where all of a sudden I just "got it", and really started to understand the power and flexibility of the Cocoa / Objective-C combination.

How did the idea for Koi pond come about?

Trost: I had come up with the idea of doing a water simulation because Brandon Bogle had done something like that in the past for one of our games. So I brought the idea to him of basically turning the iPhone into a surface of water that you could interact with using your finger.

Did you know from day one that your app would sell for