Stomata are the pores on leaves through which carbon dioxide, oxygen, and water vapor are exchanged with the atmosphere. Researchers have found that leaf stomatal densities change in response to several environmental variables, including humidity, light intensity, and atmospheric levels of carbon dioxide, a greenhouse gas (Van Der Burgh, Dilcher, Kurschner, 1993). This article describes a simple laboratory technique that allows students to explore leaf stomata as a bioindicator for a variety of environmental changes. Content knowledge contextualized by this work with stomata includes photosynthesis, structure and function of leaves, structure and function of stomata, the stomata's role in gas exchange and the environmental balance that plants must maintain between water loss (transpiration) and carbon dioxide uptake. The laboratory work also requires that students learn and use appropriate microscope skills and apply important data handling skills. (Contains 4 figures.)