Public art is never static. People, traffic, birds, squirrels, weather and time of day all change things up. Take, for example,  Ai Weiwei's 37-foot-high steel cage in Washington Square Park (until February 11, 2018). By day, it was a total selfie magnet for tourists. At night, it ruled the park. Darkness transformed Weiwei's center silhouette (modeled after a 1937 gallery doorway by Marcel DuChamp) into a beckoning giant keyhole. Floodlights on the arch turned park walkers into miniature moving cut-outs. Perhaps the conjoined couple in the cage had just stepped out to explore the world around them? I wondered if they might snap back into place at dawn, like two missing puzzle pieces.