In the case of the clay spheres, these factors would include, clay density, size, shape, surface area, and volume, as well as thermal conductivity of clay, initial temperatures of clay and of surrounding medium, in this case water. The instructor should emphasize the process of doing an experiment, namely, that all variables except one are controlled for each experiment performed. For each experiment all variables except temperature are controlled. The dependent variable in the experiment is the clay sphere's internal temperature, and the independent variable is the time over which the clay cools. For all four experiments the shape, and intrinsic properties of the clay are held constant.
Students collect temperture vs. time data for each of the four spheres.
The objective is for the students to investigate the relationship between the cooling rates and the geometric properties of the four spheres. It is important to allow enough time for the students to analyze their temperature vs. time data for each experiment. Curve fitting to the data can be a rich learning experience especially if groups are given sufficient time to discuss and analyze their data. Also a general class discussion of the results is a good way to emphasize the important results, namely, that the cooling rates depend on the surface area to volume ratios of the clay spheres. The larger this ratio, the faster the sphere cools.