Recently a colleague of mine set down his passcode-secured iPhone on the desk we were sitting at. As I was marveling at how smudged the screen was from his constant use, I noticed that among the various smudges I could clearly see four distinct fingerprints, whose positions I realized revealed the four numbers he used for his passcode lock. The passcode lock is a feature of the iPhone that, when enabled, requires the user to enter a four-digit code to unlock the phone. It’s a great feature to keep your contacts, email, and account secure should your iPhone get lost or stolen. But because you have to type in your passcode every time you use the phone, the four fingerprints over those numbers can easily become the most distinct marks among the smudges.