X-Books are for kids who love facts—especially in bite-sized portions and about fascinating, true subjects. As plentiful images and graphical representations stimulate visual perception, readers will be drawn into learning how to use informational texts. Each title is framed by a signature Top 5 countdown feature that not only serves as an example of sequential order but also narrates a brief story within the main topic: in X-Books: Marine Mammals, these sidebars summarize the physical features and unique aspects of some of the most fascinating marine mammals in history. Count down the world's most extreme marine mammals, and launch a lifetime of reading without limits! A countdown of five of the most captivating sea otters provides thrills as readers learn about the biological, social, and hunting characteristics of these intelligent marine mammals.