This study investigates how 25 junior high school students employed their bodies of knowledge and responded to problem cues while individually performing a science experiment and reasoning about a drops phenomenon. Line-by-line content analysis conducted on students' written ad hoc explanations aimed to reveal students' concepts and their relations within their explanations, and to construe students' mental models for the science phenomenon based on level of specification, models' correspondence with scientific claims, macro versus micro view of matter, and type of evidence used. We then inferred four types of knowledge representations for the nature of matter. Findings are discussed in terms of implications for science teaching.