A study described the social interactions of ability-grouped dyads as they construct knowledge of the scientific concept of balance to elucidate the relationship between interactions and conceptual growth. The verbal and nonverbal behaviors of 30 fifth-grade students were recorded as they completed three activities related to balance and levers. These student interactions were examined within a framework of social cognition. For each dyad, characteristics of ability-grouped dyads were identified. Results revealed that high students (those who scored in the upper quartile of the California Achievement Reading Test) effectively used prior experiences, maintained focus on the learning task, and were able to manipulate the equipment effectively to construct knowledge. Low students (those who scored in the lowest quartile on the California Achievement Reading Test) exhibited off-task behavior, lacked a metacognitive framework for organizing learning tasks, centered on irrelevant features of the equipment and were unable to use language effectively to mediate learning. Within low-high student dyads, high-achieving students modeled thinking processes and strategies for manipulating equipment and focused the low-achieving students on the components of the tasks while verbally monitoring their progress, thus enabling the low students to identify the critical features necessary for concept construction. These results highlight the differences that students manifest in the use of language and tools. Low students' inefficient use of tools has implications for the ways science teachers structure lessons and group students for laboratory work. (Contains 26 references.) (Author/RS)