The polymerase chain reaction (PCR) is a common technique used in high school and undergraduate science teaching. Students often do not fully comprehend the underlying principles of the technique and how optimization of the protocol affects the outcome and analysis. In this molecular biology laboratory, students learn the steps of PCR with an emphasis on primer composition and annealing temperature, which they manipulate to test the effect on successful DNA amplification. Students design experiments to test their hypotheses, promoting a discovery-based approach to laboratory teaching and development of criticalthinking and reasoning skills.