In this article, the author describes an open-ended drawing task that was used to discover young children's experiences with, and understandings of, the concept of mass. Mass is defined as the amount of matter in an object, and, like time, it cannot be seen (NSW Department of Education and Training Professional Support and Curriculum Directorate [NSW DET PS&CD], 2003). As a consequence of its invisibility, children often find the concept of mass difficult to understand. One way children can communicate their understandings about mass is by drawing, and providing explanations of their drawings. In the drawing task, children in their first year of formal schooling (aged 4 to 6 years) were asked to draw something heavy and something light, and to describe their drawing. This process, which Wright (2007) has termed "drawing-telling", captures the ways in which children make sense of the concept of mass, and identifies the prior experiences and background knowledge brought to the concept by the children. Upon viewing the drawings and accompanying stories, it became clear that the children used these prior experiences and background knowledge to construct their own personal "theories" about mass. The author discusses the four theories that were identified by the children. (Contains 4 figures.)