It happens in every city, particularly in North America: things disappear. They become other things, or sometimes they become nothing. But they disappear, either because no one wanted them, or they were dated, or dilapidated, or just plain ugly. Eventually, you walk past something that was once something else, and you can’t even remember what it used to be. And that moment, says Geoff Pevere, is one of the saddest aspects of modern urban existence.