Over the past two decades, video analyses of science classroom have developed tools to describe, model and understand teaching and learning processes. Following the activity theory approach (Leontiev, 1978) using tools adapting from communication analysis (Givry & Roth, 2006), we propose to show what are the contributions of the interplay of talk, gestures and semiotic resources in the setting to analyze students' conceptualization. Our database consists of questionnaires, interviews and classroom video data and written worksheets of 14 students at the upper-secondary school level (Grade 10 [15-year-old students]) during a one-month teaching sequence about gas. We use Kronos software to code videos through several levels : Tasks (Macro), actions (Meso), meaning-making (Micro). Based on semiotic resources contained in language, we reconstruct in great detail all the ideas about gas expressed by students during the entire teaching sequence. Our results show (a) that we need to analyze simultaneously talk, gesture and salient element of the setting to reconstruct the meaning performed by student during communication acts. (b) how the interplay of talk, gestures and salient elements could be pertinent to describe the construction and the stabilization of the meaning of the word quantity performed by a student. However, some research already shows that prosodic features of speech (intensity, pitch, pitch contours) are further semiotic resources used in classroom conversations (Roth, 2005). Future research will show whether and how such additional resources further improve our unit of analysis called ''idea''. Furthermore, our results concern only a few students and cannot be extrapolated. We can considered it as assumptions which will be tested in future research.