Glass is a transparent material in terms of light and sight, sometimes also for sound or temperature, yet at the same time is opaque in terms of smells or the movement of persons or things. In separating different environments, it simultaneously links them; it allows interactions, yet no physical contact. These distinctive features appear to allow certain forms of human interplays to unfold that otherwise would be impossible. This is particularly true in the case of private encounters, where gestures and gazes can be exchanged through the glass; where a forbidden closeness is possible, albeit devoid of all transgression; or where even, through the reflection of the glass, silent observation is possible.