Mating between closely related individuals often causes reduced fitness, which is termed "inbreeding depression". Inbreeding is, therefore, a threat towards the persistence of animal and plant populations. Here we present methods and results from a practical for high-school and first-year university students and discuss learning outcomes of the exercise as an example of inquiry-based science teaching. We use the model organism "Drosophila melanogaster" to test the ability of inbred and control (non-inbred) females to survive heat stress exposure. Flies were anaesthetised and collected into vials before exposure to 38[degrees]C heat stress in a water bath for 1 h. Half an hour later the number of comatose inbred and control flies were scored and chi-square statistic procedures were used to test for different degrees of heat stress tolerance between the two lines of flies. The practical introduces the students to central topics in population genetics and demonstrates potential consequences of inbreeding and environmental stress on fitness. Furthermore, the practical gives insight into how to design, analyse, and interpret the results of a scientific study. (Contains 4 tables and 3 figures.)