"Untitled", Larry Bell. Vacuum-coated glass
A few years ago, on a trip to Washington DC, I took a walk through the Hirshhorn Museum's sculpture garden. There was a cube there, highly reflective, chest-high, beautiful. The piece stuck with me for its simplicity and the beautiful way that it echoed its surroundings.
Fast forward to September 2010. I'm walking through the Oakland Museum of California, and stumble across the piece above. I know it instantly. It's not the same piece, but I know it's by the same artist.
This piece reflects its surroundings, yes. But more magically, from certain angles, you can also see THROUGH the cube. Forward and backward at once.