Themes: Wormholes, Space, Nonfiction, Tween. What are wormholes? What causes black holes? How are black holes and white holes different? Can people travel in time and across space? Where would you go if you could time travel? Red Rhino Books Nonfiction is the first Hi-Lo children's nonfiction series at a first-grade reading level for students in grades 4–7. Visually appealing fullcolor photographs and illustrations—along with fun fact boxes—keep these books flowing. Also includes prereading comprehension questions along with a 20-word glossary for further reading support.