Traces of interfaces.

It’s hard to appreciate the variety of UIs though, since turning the screen off removes virtually all evidence of them. To spotlight these differences, I looked at the only fragments that remain from using an app: fingerprints.
Remnants of a Disappearing UI via things magazine

I often play Boggle on my iPod touch during my commute and end the day with a four-by-four grid of smudges clustered around the home button.