This lesson is one of a series involving the geology of Grand Canyon National Park. Students are asked to make a model of the phenomenon of erosion as it relates to the landscape formation of the Grand Canyon. Students use a milk carton designed to allow water flow through its spout in order to simulate a flowing river. Topographical changes in elevation are demonstrated as students raise the height of the carton as water is flowing. Other variables include type of sediment used in the simulation. Students observe ways water flow is affected by sediment choice (sand/potting soil) and the degree of its saturation.