Dave's Down-to-Earth Rock Shop (Classifying): Budding geologists Josh and Amy are crazy about collecting rocks. And with the help of local expert Dave, they learn how to sort rocks by different attributes: size, color, hardness and type. "We're kind of like rock detectives," says Amy. The story is filled with lots of rock facts. Classifying objects according to attributes is a skill used throughout mathematics and science. (Dave's shop is a real place, located in Evanston, Illinois, around the block from Stuart's old house.) Illustrated by Cat Bowman Smith.