What I did one summer three years ago has shaped my life ever since.
I was 17 and armed with my first-generation iPhone. I had been trying to get into iPhone development ever since the SDK came out about a year prior but hadn't really got anywhere due to school. Summer came and I took it as an opportunity to buckle down and get something on the App Store.
I got about halfway through Beginning iPhone Development and decided I was capable of a basic table-based app. Naturally, what came to mind was a database of....brocabulary. Some friends and I were going through a period of dropping these whenever possible and it was getting awfully hard to remember all of our permutations. What else is a hacker to do but solve their own problems?
Spent about two weeks coding up the app and brainstorming with my friends for the ~100 initial terms. Submitted to the App Store around the middle of June (right around the time OS 3.0 was released) and was released a week later under the title Brocabulary. Got some downloads, made a bit of money ($0.99 per purchase), and was instantly hooked on making apps.
I mean really, how awesome was it that I could make something in my bedroom that a) made money and b) complete strangers told me they enjoyed?
So that was my first few weeks of summer. For my next project I decided that I wanted to do a game, albeit something simple enough that I could actually ship. Most iPhone games used the accelerometer for their functionality, but I wanted something more unique. I looked at all the features of the iPhone and decided the microphone was underrepresented on the App Store. I came up with the idea of blowing into it to push a paper plane.
The crucial part was figuring out how to measure the input of the microphone, which I adapted from some code I found on StackOverflow. I utilized the relatively nascent Cocos2D2 game engine to make life easier. At the time I didn't know any backend technologies, so I went with the now-defunct scoreboard hosting service CocosLive to store how long players "blew" for. After about another two weeks of work, Paper Planes was ready.
It went live on the App Store in mid-July and got something like 1,000 downloads on the first day. That was HUGE to me, since Brocabulary had only gotten ~200 downloads in its entire (young) life. Immediately I sent in an update which placed an ad on the scoreboard screen, in hopes of getting some small revenue ;) (in the end I wound up with a couple hundred dollars). Since then, it's gone on to get about a million downloads during it's lifetime.
I worked on some other apps over the remainder of the summer but I feel like this covers the important parts.
I went from merely taking computer science classes in school to making products that shipped to thousands of people, all in a matter of months. From then on I knew there was nothing quite like software for me. In retrospect the code was rough and the apps were a little buggy, but a kid has to start somewhere right?
I wrote about my experience making apps in all my college essays, and I think it played a big part in my admission to UC Berkeley. I got out to California and dove into the hacker scene, using my iOS skills as an inroad. One thing led to another and suddenly I'm cofounder of a Y-Combinator startup, making an iPad app. I've gone on to help build beautiful iOS and Android apps (Circle), and am now a Thiel Fellow working on app-related tools.
Without that one summer learning iOS development, none of that would've happened.
This was originally an answer to a Quora question