This book uses childrens' natural ability to classify objects to introduce the concept of a set and membership in a set. Sets are fundamental to modern mathematics. Every principle of number theory, arithmetic, algebra, geometry, trigonometry and calculus is defined in terms of sets. An early understanding of the principles, language and notation of sets will prepare a child to absorb higher math concepts quickly and thoroughly. A definition of a set is "a collection of objects where it is known if any particular object is a member of the set." A 'collection' is any grouping. An 'object' can be anything; a toy, a number, or even another set. 'Membership' is a critical concept here. To form a set, one must be able to tell exactly what is in the set and what is not in the set. Each set must be clearly defined. This book starts with a definition for the types of objects that can be included in a set and a definition for each set. "A swing set is a group of swings that hang from the same pole. A swing is something that swings back and forth that kids can ride on." This definition is emphasized throughout the book. As you discuss this book with your child, emphasize the definitions given and whether objects not found in the book meets the definitions. This book teaches the child how to form a math statement such as f is an element of B from a situation involving sets. This book also teaches the child how to translate a math statement expressing membership in a set into words.