This unit of the Flexible Learning System (FLS), the first of a 3-volume series on children's thinking, discusses the development of classification in children between 3 and 8 years of age. The series is based on the application of Jean Piaget's work to early childhood education. The development of classification is revealed in the way children use judgments about similarities and differences and the concepts of "some" and "all." The unit is designed to be used with a group of adults to help them understand class and its organization in the preconceptual, intuitive and concrete-operational stages of development. Practice in exploring children's thinking is provided. Activities include thought problems, role playing, child interviews, discussions, readings, and viewing a color videotape on classification from a series entitled The Growing Mind: A Piagetian View of Young Children. Rip-out guided interview forms are provided for all interviews. Also included are an introduction to Piaget and his general theory, educational implications of the theory, classification materials, a transcript of the videotape, and an annotated bibliography. Related FLS units exploring children's thinking: "Exploring Children's Thinking: Seriation"; "Exploring Children's Thinking: Conservation"; "Working with Children's Concepts"; "Using Toys and Games with Children"; "Developing Children's Sense Perception." (Author/SB)