Through observation of fifth-grade mathematics and social studies classes, this study reveals that subject matter, a variable often overlooked in recent research, has a profound effect on instructional practice. The study analyzes the interrelations among forms of instruction, levels of student involvement, and subject matter. The analysis challenges educational research showing that classroom activities are coherent actions shaped by the instructional context--especially what is taught. The research contradicts the received view of both teaching and learning as uniform and consistent arguing that individual teachers arrange instruction very differently, depending on what they are teaching, and students respond to instruction very differently, depending on the structure and demands of the lesson. (RJC)