The home botton of the iPhone has some magic and you can be a wizard if you know the secrets.
Secret #1: Awake
If the iPhone is sleeping you can click the home or lock button to awake it.
Secret #2: Get home
At any point you can go back to the homescreen if just click the home button.
Secret #3: Screenshot
Do you have something interesting on the iPhone screen and would like to share with your friends? Then take a screenshot by holding the lock button and the home button together. On success the screen flashes shortly white and the taken picture is in the iPhones Photo gallery.
Secret #4: Quit application
An application hung off? When holding down the homebutton for 6 seconds, the current app will be forced to close and you get bumped back to the home screen.
Secret #5: Reboot / Shut down
Sometimes it even makes sense to reboot a Apple product. If you think it does react slower as used or apps are closing by themselfes unusally often, you are having connection problems, etc., then force a shutdown. Hold the lock button and the home button together for about 10 seconds. On the top you will see a slider and if you move it, the iPhone shuts down. Once it is powered off, just turn it on by clicking the home button or the lock button.
Secret #6: iPod control
If you are listening to music with the integrated iPod, it is sometimes unwanted to leave the current running application only to go to the next track or pause. Of course it is easy when a headphone is connected, because it can do those functions without touching the iPhone. But, if no headphone is on? Just double tap the home homebutton! Then you will see a simple interface for the most important play functions. Rember, this does only work when iPod was lauched before and is playing music.

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