For the most part, you never have to power down your trusty iPhone, but there are two rare situations where doing so might be useful. If your battery is getting low and you won’t be able to charge before you absolutely need the phone later in the day, turning it off will prevent it from running out of juice. Also, if your iPhone is being weird (it is a computer, after all!), power cycling can reset its little iPhone brain. To pull its virtual plug, press and hold the Sleep/Wake button until you get the necessary Slide to Power Off control. When it’s time to turn it back on, press and hold Sleep/Wake again until you see the Apple logo on the screen.