There is a continuing need for engaging inquiry-based laboratory experiences for advanced high school and undergraduate biology courses. We describe a guided-inquiry exercise investigating the pH-dependence of lactase enzyme that uses an inexpensive, wide-range buffering system, lactase dietary supplement, and over-the-counter glucose test strips. Learning-outcome assessments indicate the laboratory effectively teaches biology content as well as experimental assay design, and that students appreciate learning new skills as well as the challenge and rewards of designing their own experiment.