The new iPhone 5S comes with a built-in biometric scanner that lets you unlock it with your fingerprint. But that's not the only body part you can use to unlock your phone.
People have been testing out the limits of the scanner and have found you can set it up to unlock with your TOE . . . or your NIPPLE. That's not practical, but it's doable.
It can actually distinguish your nipples from other people's nipples pressed up against the sensor.
Your nipples don't have as unique of a print as your finger, but the sensor also takes a biometric reading . . . and it apparently gets that through your nipples.