The Water and Molecular Networks (WAMNet) Project uses graduate student written Reduced Instruction Set Computing (RISC) computer simulations of the molecular structure of water to assist high school students learn about the nature of water. This study examined: (1) preconceptions concerning the molecular structure of water common among high school students; (2) the effect of making and testing predictions using visual, interactive computer simulations on students' conceptions of the microscopic properties of water; and (3) aspects of the simulations that were most helpful in promoting conceptual change. Ten students from high schools in the Boston, Massachusetts area participated in 50 hours of individual and pair demonstration interviews in which students were asked to think aloud and interpret what they saw on the computer screen as they used the computer simulations. Analysis of transcriptions of the interviews indicated that: (1) students had preconceptions concerning the composition of individual water molecules and the structure of molecular water in ice, vapor, and vapor forms; (2) student concepts of water composition, the molecular structure, and the relationship between the kinetic energy of particles and their temperature changed as a result of working with the water simulation; and (3) visualization, multiple representations, interactive control of parameters, and debate with partners contributed to that change in the students' concepts. Contains 14 references. (MDH)