This paper presents the results of an experiment which investigated elementary school children’s conceptual knowledge about the earth. First-, 3rd-, and 5thgrade children were asked a series of questions about the shape of the earth. Children’s responses to these questions revealed considerable apparent inconsistency. For example, many children said that the earth is round but also stated that it has an end or edge from which people could fall. A great deal of this apparent inconsistency could be explained by assuming that the children used, in a consistent fashion, a mental model of the earth other than the spherical earth model. Five alternative mental models of the earth were identified: the rectangular earth, the disc earth, the dual earth, the hollow sphere, and the flattened sphere. It is argued that these models are constrained by certain presuppositions which children form based on interpretations of their everyday experience. Some of these models (the rectangular earth and the disc earth) seem to be initial models children construct before they are exposed to the culturally accepted information that the earth is a sphere. In the process of knowledge acquisition, children appear to modify their initial models to make them more consistent with the culturally accepted model by gradually reinterpreting their presuppositions. Synthetic models (such as the hollow sphere and the flattened sphere) are generated by children The purpose of this paper is to investigate the development of children’s conceptual knowledge about the earth’s shape. We are interested in understanding the nature of children’s initial knowledge about the shape of the earth and in finding out how this knowledge changes during the elementary school years as children are exposed to the culturally accepted information that the earth is a sphere.