In this paper we describe a product dissection activity that has been developed for a graduate course on product family design to improve students' understanding of platform commonality. Last spring, the product dissection activity served a second purpose, namely, it provided an opportunity to engage students in product family design research in the classroom by having them participate in a study to evaluate the variability in the Product Line Commonality Index (PCI), a commonality index from the literature. The product dissection activity consisted of five teams dissecting and analyzing three different families of products, each containing four products. Based on their results, we identified three main sources of the variability that occur during the dissection of the products and calculation of the PCI: different levels of dissection, parts omitted from the analysis, and different values for the factors used to compute the PCI. Recommendations for reducing the variability are given based on our findings. Finally, an assessment of the students' learning reveals that the activity significantly improved their understanding of platform commonality.