What happens when a museum lets six distinguished artists into its store room of 20th-century art, asking each to curate an exhibition from works they find there? Head to the Guggenheim to find out. Cai Guo-Qiang, Paul Chan, Jenny Holzer, Julie Mehretu, Richard Prince, and Carrie Mae Weems are the chosen artists. Their exhibitions are one to a floor. Sly commentary on curation, on museum biases, and on aesthetic branding are discernible. —L.J.