Imagine laying in the grass behind your grandparents' house on a fall afternoon, staring at the sky and determining the shapes of the clouds as they pass overhead. Is that cloud a turtle or a dragon?

George Seurat's idea of relaxing on a Sunday afternoon, "A Sunday on La Grande Jatte" (1884), may look pretty crowded in this socially-distant world of ours. But it is one of the most reproduced works ...