Because density is unobservable and must be inferred from knowledge about weight and size, it is a difficult concept to teach and learn. Even after traditional instruction, many students still have an undifferentiated concept that mixes characteristics of both weight and density. In this study, researchers tested the effectiveness of a unit they created to help students make this difficult conceptual differentiation, which is crucial to understanding the particulate nature of matter. The Educational Technology Center's Weight/Density Unit uses both computer simulations and classroom activities with real materials of different weights, sizes, and densities. The simulations attempt to make density more visually accessible than it is with real objects. Researchers used the unit in one sixth-grade and one seventh-grade class. The findings suggest that providing conceptual change is both difficult and possible, and they further suggest that computer models, used in combination with hands-on materials can help students to understand an abstract and perceptually inaccessible concept such as density. Evidence from the pre- and posttest and the clinical interviews suggests that the teaching intervention brought about two kinds of change: (1) conceptual differentiation among students who initially made none; and (2) conceptual consolidation in which students who already had a beginning, fragile distinction deepened and extended their understanding. Appendixes make up the bulk of the volume. They include samples of the assessment instruments; teaching materials for three units, with worksheets; and a discussion of the computer programs. (CW)