Why is it so hard to build stable sand castles with dry sand? Page 35 reveals the engineering basis that can be applied to other "construction" projects.BioFables Companion Books are designed to help parents and teachers discover new insights and a deeper understanding of Sand Sack's lessons that might not be apparent on first reading. The color-coded, annotated Knowledge Pinwheel(TM) and the Discovery Paths(TM) identify major subjects and topics to help you visualize important categories of learning. Use the supplemental narratives to spark conversation with young readers.