Beyond the basics, Apple has said little of how the iPhone 5s Touch ID fingerprint reader works -- we mostly know that it's inaccessible to the outside world. Thankfully, the company has shed further light on Touch ID through statements to the Wall Street Journal. To start, iPhone owners will have to unlock with a passcode if they either reboot or haven't unlocked within 48 hours. The safeguard prevents hackers from simply biding their time while they look for a workaround, Apple says. Legitimate users will also want to keep their hands dry, as the reader doesn't work well with fingers covered in sweat and other liquids. You won't want to try unlocking immediately after running, then, but it's evident that Apple already knows many of Touch ID's real-world limitations.