This analysis follows students' action and interaction with a single scientific phenomenon (bubbling/gas) over the course of a curriculum unit in a middle school science classroom to examine how and what they learn when doing laboratory activities. Taking a situated approach to interaction, I place the process of objectification in its multimodal context, focusing specifically on perceptual and linguistic objectification to highlight the importance of action, bodily movement, and the use of materials in students' developing understanding of the law of conservation of matter. These two processes of objectification show how students orient themselves to the scientific materials at critical points in time, and how they transform first-hand activity into meaningful linguistic representations to demonstrate they have learned from their activities, and to create forms of knowledge that can be decontextualized and recontextualized in future learning situations. (Contains 4 figures.)