I once wanted to try using the backs of iPods as part of a keyless entry system. We all carried iPods anyway, and everyone's iPod had a different scratch pattern on back, so have a system where you hold your iPod back up to a camera and enter a PIN. The scratch pattern tells the system who is requesting entry. If they enter the PIN for that person let 'em in.

Good job you didn’t need to look ~~at~~ through it all day in order to use the product then!