Humans do math or at least some humans can do so. New research shows that to prevent starvation at night, plants perform accurate arithmetic division. The calculation allows them to use up their starch reserves at a constant rate so that they run out almost precisely at dawn. Plants feed themselves during the day by using energy from the sun to convert carbon dioxide into sugars and starch. Once the sun has set, they must depend on a store of starch to prevent starvation.