Ocean acidification is one of the most serious environmental issues facing the planet (e.g., Doney 2006; Guinotte and Fabry 2009). It is caused by excess carbon dioxide (CO[subscript 2]) in the atmosphere. Human activities such as burning fossil fuels put CO[subscript 2] and other heat-trapping gases into the atmosphere, which causes the Earth's average temperature to increase. This increase is commonly referred to as "global warming" or "climate change," but the problem doesn't stop there. The two lessons presented here familiarize middle school students with the causes and consequences of ocean acidification. They also provide students with an opportunity to generate hypotheses, collect data through hands-on experimentation, and analyze results. These lessons can be successfully integrated into Earth science, biology, environmental science, marine science, or chemistry curricula. (Contains 6 figures.)