This study explores middle and high school students' understanding of the particulate nature of matter after they were taught the concept. A total of 87 students (41 high school and 46 middle school) participated in the study. Findings suggest that students held misconceptions about the law of conservation of matter, chemical composition of matter in different phases of matter, the process of condensation, and behaviors of molecules at a microscopic level. The discussion focuses on the implications of these findings for enhancing science teachers' pedagogical content knowledge.