Earth's surface is constantly changing. Weathering, erosion, and deposition break down Earth materials, transport those materials, and place them in new locations. Children see evidence of these processes all around them. The sidewalk or playground surface cracks and has plants growing in it. Pieces of a rock wall or the sides of a building break off and crumble. A beach may look different after a storm than it did before. Some of those changes happen quickly, while others take hundreds to thousands and even millions of years. "A Framework for K-12 Science Education" states that elementary school-age children should learn about the development of landforms and how they weather and erode in order to understand that Earth changes over time (NRC 2012, p. 178). The "Framework" identifies a number of scientific practices in which students should regularly engage. One of those is the construction and use of models. Models are particularly beneficial in Earth science as we can make small-scale models that represent geologic processes that occur on large scales. We can also speed up processes that normally cannot be observed in human time frames. Creating and using models helps children construct evidence-based explanations (another scientific practice identified by the "Framework") for the ways in which Earth's surface changes over time. This lesson is designed for fourth graders, but could be adapted for third or fifth graders. As children construct a stream model to simulate erosion due to water, they investigate factors that affect the rate at which water erodes land. Because water plays a major role in erosion in all climate types, these lessons can be adapted to different locations. This investigation would most appropriately follow lessons about water's role in mechanical and chemical weathering.