Gr 1-3--Best friends Katy and Micah begin their volunteer assignment at Mountain Bluff Zoo tasked with the question of why Charlie, one of three capuchin monkeys, is listless and overweight. Charlie eats the same food as his enclosure mates Willie and Wanda, but as the other two monkeys play, Charlie returns to his perch and falls asleep. Katy, who is Black, likes to draw and is two years older than her friend Micah, who is white and likes to take pictures. Katy's mother is a zookeeper. The two animal-loving friends make a good team: When Micah dozes and expresses pessimism after their first day, Katy admonishes him. Micah then researches capuchin monkey facts on his own, and both come up with suggestions for new ways to enrich Charlie and the other monkeys' enclosure. Using simple sentences and repeated phrases, this science-minded mystery highlights the importance of observation and patience. Black-and-white cartoon illustrations add details that enhance the plot and limn the characters. This is a good option for animal lovers who are not quite ready for Patricia Reilly Giff's longer "Mysteries on Zoo Lane" series. VERDICT Emergent animal-loving readers will find this zoo mystery engaging, with its focus on simple STEM concepts and relatable characters.--Marilyn TaniguchiCopyright 2022 School Library Journal, LLC Used with permission.