Scientists often model complex biological phenomena in vitro, mimicking conditions found in living organisms. Understanding the power and limitations of biological models is an important topic in undergraduate science. In this activity, students develop their own in vitro model for testing the survival of bacteria from commercial probiotic supplements. Students work in groups to decide which factors are important for survival of bacteria in a chosen portion of the human digestive tract. Groups of students create their own in vitro models of organs such as stomach and/or intestines. Students expose a probiotic supplement to conditions mimicking the chosen portion of the human digestive tract, and measure the effect of those conditions on the survival of bacteria found in the supplement. Students choose to focus on conditions such as low pH found in stomach or pancreatic enzymes found in the upper intestine. Through this activity, students gain experience with serial dilutions and calculations of colony forming units (CFUs). This project also provides the students with the valuable experience of designing experiments in small groups. Students present their findings in a poster session, which provides a venue for discussing the validity and limitation of various models.