A software tool called 3D-LAB has been developed for learning and teaching three-dimensional geometry. With this microworld, educators and students can display three dimensional solid objects, rotate them, modify them, open them up, draw points and segments, and measure lengths, areas, volumes, and angles. The major characteristics of this tool are the interactivity of the manipulations and the variability of the objects. Two conflicting aims--ease of manipulation and variety of manipulable solids--are realized by a combined method of loading solids first and modifying them next. Both quantitative and qualitative observations can be made using 3D-LAB. Reproductions of computer screens illustrate the discussion. (Author/BEW)