The purpose of this study was to examine the interaction between children's observations of light and shadows and their developing theories in the context of a series of lessons in a third-grade classroom. Detailed analysis of videotapes of the children's discussions of their investigations into light and shadows led to the finding that the children's observations of light led to the development of a limited theory about light that was largely useful in explaining classroom investigations. However, this theory of light was less helpful in helping them understanding shadow phenomena and was too limited to explain out-of-classroom observations of light and shadows.