Prior research on collaborative learning identifies student behaviors that significantly predict student achievement, such as giving explanations of one's thinking. Less often studied is the role of teachers' instructional practices in collaboration among students. This article investigates the extent to which teachers engage in practices that support students' explanations of their thinking, and how these teacher practices might be related to the nature of explanations that students give when asked by the teacher to collaborate with each other. The teachers observed here, all of whom received specific instruction in eliciting the details of student thinking, varied significantly in the extent to which they asked students to elaborate on their suggestions. This variation corresponded to variation across classrooms in the nature and extent of student explanations during collaborative conversations and to differences in student achievement. (Contains 9 tables.)