Teachers at the Freeburg Early Childhood Program know that experimentation with physical science is of great interest to young children, and can begin as early as the age of 3. The constructivist teachers at this experimental school at the University of Northern Iowa worked for six years to develop a center-based approach to physical science with young children. Kamii and DeVries (1993) define physical knowledge activities as those in which children experiment and figure out how things work in the physical world. In these activities, children act on objects to produce desired effects and observe the objects' reactions. In this article, the authors illustrate how such activities can be implemented in the classroom to foster young children's thinking and reasoning.