Students tend to have a poor understanding of the concept of gas pressure. Usually, gas pressure is taught in terms of the various formulaic gas laws. The development of the concept of gas pressure according to the early Greeks did not include the concept of a vacuum. It was not for another 2000 years that Torricelli proposed that a vacuum can exist and that he was able to produce a vacuum above a column of mercury. However, the existence of a vacuum continued to be a contentious issue for at least another 100 years. During this time the behavior of gases was studied by Boyle, Amontons, Gay-Lussac, Daniel Bernouli, Charles, and Dalton. In the 19th century gas behavior was revisited and studied from the molecular level through the work of Graham, Maxwell, Boltzmann, and van der Waal. The stories of conflicting theories and the development of the concept of gas pressure gives students an increased appreciation for the nature of science and helps them with conceptual understanding of the concept of gas pressure.