This study examined the effect of an innovative teaching activity to improve concrete operational thinking skills with preschoolers in Head Start programs. A "learning set" of classification games and seriation games was used to teach the oddity principle and insertion into a series. These games were played with the children using toy ponies and hand puppets as props for a period of 4 months with 15 Head Start 4-year-olds. At the conclusion of this form of instruction, the children were significantly better than a comparison group of Head Start children at both classification and seriation. This superiority extended from problems involving three-dimensional objects to two-dimensional representations of oddity and seriation problems. The children's improvement has positive implications for transition from preschool to grades K-3, as well as for improving skills of at-risk children. (Contains 13 references.) (JPB)