The iPhone 3G goes to sleep by default after it’s been inactive for a certain period of time (you can change this period in the Settings option on your Home screen). That’s just putting it to sleep, though. In some circumstances – like a very low battery – you may want to actually turn the iPhone off.
To do so, hold down the sleep/wake button at the top right-hand corner of the iPhone until a slider appears at the top of the screen.
This screen will ask you to run the slider across the screen to turn the phone off. If you don’t want to do this, just hit the cancel button at the bottom of the screen.
When you slide the slider to the right, a progress wheel appears and in a second or two, the iPhone 3G will be off.
To turn it back on, hold down the sleep/wake button until the Apple icon appears on the screen.
