In Gravett’s gifted hands, an old math problem springs to life—and more life and more life! Medieval mathematician Fibonacci’s “rabbit problem,” in which bunnies breed at a specified rate, provides the structure of this glorious faux–wall calendar that watches a rabbit community from January to December. The title phrase alters monthly to portray current conditions in Fibonacci’s Field—for example, rainy April is The Soggy Rabbit Problem. Gorgeous art uses watercolors, oil-based pencil and carrots. The bottom calendar pages feature notations (“check babies for fleas”—“are fleas edible?”) and beautiful, bountiful paper layerings, some with complex depicted overlaps, others physically real—a newspaper in July’s Bored Rabbit Problem and a cookbook for September’s Too Many Carrots Problem have mini-pages that turn. Readers needn’t care about the math of Fibonacci Numbers to love the hilarious, jam-packed visual details, many of which are playfully metatextual. In a magnificent pop-up climax, the rabbits gnaw a hole in December’s page and explode out of the teeming field. Endless fun to pore over for kids and math-minded or geeky adults. (Picture book. 4-9, Adult)