Students are almost universally interested in animals, and especially endotherms, including mammals and birds. According to Bergmann's rule, endotherms that live in colder climates at higher latitudes are larger than those living in warmer climates. As with most biological principles, hands-on investigation will provide a better understanding of why size is important in endotherm thermal regulation. One easily observable aspect of this principle is that larger organisms have a lower ratio of body surface area to total body volume. This affects how efficiently they can retain or radiate heat, which can be easily tested in the laboratory using commonly available materials. In this activity, simple models of endotherms of different sizes are used to assess the effects of body size on heat loss.