This article describes an inquiry-based activity for high school students in which they determine the pH of the digestive compartment in "Paramecia" using different pH indicators. This lab activity introduces students to the challenges of research on the cellular level and illustrates one of the primary methods that scientists use to measure the concentrations of molecules inside cells and organelles. Furthermore, students are exposed to the essential features of inquiry because they must generate the evidence, develop explanations based on that evidence, and justify their explanations. This activity can be presented in different ways depending on how much information the students are given and how much guidance the teacher wishes to provide. This activity can be used at the beginning of the unit on cells to introduce the concept of a cell, but it can also be used in a more traditional way to support other lessons on the cell. One can introduce the concept of pH, connect that with students' prior knowledge, and then introduce the question of pH on the cellular level. (Contains 3 figures.)