This study examines thought, language, and related action in sixth-graders working to understand concepts related to mass, volume, and density. Students from a heterogeneous urban classroom worked in various social contexts including individually, in pairs, in groups of four, and as a class. Their work was examined via videotapes, field notes, written work and group products, conceptual tests, and clinical interviews. Two groups of four students formed the target population that was studied closely within the context of the larger class. Research questions focused on how concepts develop across public and private arenas, and on ways the teacher's "privileging" of particular ideas, strategies, and actions played into the students' overt goals, their use of mediational means, the connectedness of sequences of discourse and action, and the standards they employed for determining the acceptability of claims made by their classmates. Concepts developed in stages, across social arrangements, as students focused on developing techniques, making observations, finding patterns in data, and developing explanations. Teacher privileging greatly influenced standards and the strategies students employed even when the teacher attempted to focus decision-making around consensus. This study ties sociocultural and cognitive research approaches. Findings indicate that group work does not adequately support already marginalized students who have difficulty entering into group negotiations in meaningful ways. (Author/PVD)