Weathering--the physical and chemical breakdown of geologic materials--and erosion--the transport of materials by wind, water, or ice--can be subtle, yet powerful forces. For example, shale, a rock made of mud-sized particles, is by far the most common sedimentary rock, a testament to the ability of weathering and erosion to take a rock and reduce it to particles too small for us to see with our naked eye. Weathering and erosion operate on a scale of crystals and grains over tens of millions of years, and over distances of thousands of kilometers. These subtle effects are difficult to convey to students. This article presents an activity in which students can use a rock tumbler to gain a personal and scientific perspective on weathering and erosion. Through this activity, students have the opportunity to demonstrate their understanding of these concepts and work with geologic materials as they make predictions and measurements, record data, and do simple calculations. (Contains 2 figures.)