Emails often contain tiny 1×1-pixel transparent or hidden images called "web beacons" used to track when the message has been opened. I gathered all 12,383 of these pixels in my email inbox and deleted mail folder. They were etched into a piece of clear acrylic and shown leaning against the wall, an accumulation of invisible yet real objects.
So many web technologies are hacks that transform the experience of being online in ways that are both hidden to the average user, and yet are easily seen if one knows where to look. Like cookies and Google Analytics code, web beacons are invisible signals, hiding in plain sight and transmitting information about how you use your computer.