The purpose of this study is to examine how the reasoning of elementary school students develops and progresses within a learning progression. In order to do this, we analyzed third, fourth, and fifth grade elementary school students' levels of reasoning about phenomena related to changes of state of water. The results show that higher grades include higher proportions of students who showed higher-level reasoning. Every student explained the phenomenon at the macroscopic scale; however, fifth grade students considered more elaborate factors and the relationships between those factors than third and fourth grade students did. In addition, students reasoned contradictorily that the weight would increase in situations of both melting and freezing. Finally, we discuss how science curriculums in each grade can be structured as a progression in reasoning and how to provide appropriate science instruction in terms of students' long-term progression of reasoning.