Explaining the weight of objects: Why are smaller objects not always lighter?

At the start of grade 3, very few children tried to explain the difference in weights of objects by raising the possibility that they were made of different materials. (Both objects were covered with white contact paper so they could not see what material they were made of.) They were more inclined to suggest that one cylinder might be empty while the other was filled or that the two cylinders might contain different objects inside.

By the end of third grade, most Treatment children raised the possibility that the objects were made of different materials. However, some over-generalized, appealing to differences in material, when the difference in volume would suffice. And when comparing objects of different sizes but the same weight, many students concluded that the fact they had the same weight showed they were made of the same material. By the end of grade 4, the Treatment students were making considerable progress on this and other problems in invoking the idea that some objects were made of heavier kind of materials than others and in explaining how a smaller object, made of a heavier kind of material, could equal the weight of a larger object made of a lighter material. In contrast, the Control students were not. Although they did increasingly consider that objects might be made of different materials, they did not increasingly focus on some being made of “heavier kind of materials” or co-ordinate “heaviness of kind of material” with volume in explaining why things were the same weight.