A few months ago, while walking home from the subway in my Brooklyn neighborhood, I noticed a change in the sidewalk — four of the white cement paving stones had been replaced with darker, bluish-gray stones.There had been a lot of construction in the area, and at first I thought they were simply new stones, not yet faded to match the surrounding sidewalk. But when I got closer, I saw they formed an artwork, engraved with the silhouette of a young, leafless sapling. The etching was meant to approximate the shadow of a nearby street tree, although that tree, now in full leaf and several feet taller, was throwing its noticeably longer shadow in the opposite direction. A text at the bottom of one of the paving stones revealed the name of the tree, the date and time when the tree shadow was cast on the sidewalk, and the work’s title: Timecast. The tree I was looking at was two-and-half-years older than its shadow, and much changed.