By thinking about the concept of density and taking into account the research on children's ideas about this concept, the authors were able to unpack the typical sink or float activity and realize that it has students unscientifically making comparisons between objects by changing two independent variables (mass and volume) at one time. With this realization, they were able to modify the activity so that students were making comparisons in which they were only changing one independent variable (mass). The end result went well beyond the common "prediction" objective found in the classic sink or float activity. (Contains 3 figures.)