This paper examines students' mathematics learning in a social context as two versions of an instructional sequence designed to facilitate students' development of a conceptual foundation for place-value numeration are enacted in two third grade classrooms. The same instructional sequence is used in each classroom with the regular classroom teacher conducting the instruction in one class and an expert graduate student conducting instruction in the other class. Other differences between the two teaching experiments are that in the second situation, there is a deliberate focus on notation and symbolizing as well as a series of computer microworlds which are used to facilitate imagery of the instructional scenario of a candy factory. Analysis of the data indicates that the candy factory imagery both supports and constrains the mathematical conceptions the children develop. A critical difference between the two teaching experiments is the manner in which the use of imagery is constituted. The use of the computer microworlds enables more children to participate in a meaningful way in the activities and their understanding of place-value is enhanced as a result. Contains 19 references. (Author/DDR)