Sources
The idea for this lesson came from Jen McGonigle, a science teacher from Haddonfield, New Jersey, who shared this activity during the Exploratorium’s Summer Teacher Institute.
Wikipedia has a wonderful summary of the tragedy of the commons. It includes an excellent review of Hardin’s original essay on the topic, historical background about where the concept of the commons originated, and many examples of modern commons problems: littering, air quality, water quality, population growth, logging, etc.