Homeostasis, the control of an internal environment to maintain stable, relatively constant conditions, is a key concept in physiology. In endothermic species, including humans ("Homo sapiens"), the control of body temperature is fundamental to the control of a suitable internal environment. To help regulate core body temperature, the body has a number of different responses. For example, under warm conditions by initiating processes such as peripheral vasodilation and sweating, heat is dissipated via the skin's surface to maintain a constant core temperature. The activity described in this article seeks to illustrate the difference between thermal perception (how warm or cold individuals "feel") and core body temperature by monitoring changes in both during exercise. Using exercise, the activity can change thermal perception quickly, although core temperature will remain relatively stable. While homeostasis remains a core theme in physiology, many aspects of it are difficult to illustrate in a noninvasive activity (e.g., ion concentrations). Temperature can be more easily manipulated to demonstrate the concept, and the presence of the carotid artery, just behind the ear, ensures that core body temperature is easy to measure. As a relatively simple activity, with participation through exercise from the participant, this is ideal for science festivals and fairs and has the potential to be used as a quick demonstration in the classroom.