Regular use gets your iPhone dirty, even when it is simply placed in your pocket and carried around. As you use it, any dirt, oils and grime from your hands can stick to the buttons and make them dirty. The grime can cause problems with the phone functioning properly. You can clean the lock/unlock button -- also known as the sleep/wake button -- on your own. If it sticks or does not work properly after cleaning it, you may need to take it to an authorized Apple dealer for further service.
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Disconnect all power cables from your iPhone. If your iPhone is plugged into your computer, unmount it so it can safely be disconnected before you remove it from the docking station.
Hold the sleep/wake button down and push the red slider across the screen to power the iPhone down. The iPhone is at much lower risk for damage while you are cleaning it if it is off. Allow it to turn off completely.
Dampen the corner of the microfiber cloth with distilled water. Do not soak it. Rub the damp corner on the sleep/wake button on the iPhone to remove the grime. Examine it to see if it is clean. Repeat until the grime is gone. Clean the rest of the buttons and outside of the iPhone the same way.
Wipe away any extra moisture with the dry part of the microfiber cloth. Power the iPhone back on.