With all this sprinting, where are your users rushing off to? It's often to another app. When you're engrossed in the design of your own app, it's naturally the center of your attention, and it's easy to imagine that it will be your audience's center of attention, too: for them, it will no longer be an iPhone, it will simply be a device for running Acme SuperNotepad. As an iPhone user yourself, you know better. Every app is just one among many, a character in a big dramatic cast of which you are not the director.
Not only will people hop away to other apps, but those other apps can and will interrupt yours with push notifications. Phone calls will ring in and text messages will saunter through. Users will also expect to share content from your app with other apps and possibly vice versa. For app designers, this means you have to think about your app not in isolation but as part of a community of neighbor apps that will share space, communicate, and occasionally step on each other's toes. (Chapter 11 explores how your app can mingle with the crowd and avoid being the antisocial guy in the corner.)
This noisy throng of apps on your audience's iPhones means that you have to think crisply about your app's role at this party. The best apps have a focused job description. The more tightly you define the idea for your app, the clearer it will be to your audience when and why they should use it. Think of the iPhone as a toolbox with lots and lots of specific tools. The "right tool for the right job" rule applies here. When you assume that people will have lots of other tools in their kit, that means your app doesn't have to do everything. Choose an idea, focus it, figure out the minimum your app has to do to make it happen, and then polish, polish, polish. You'll learn more about focusing your app in the next chapter.