Filter dirty water and then measure the turbidity of the water. We gave the students the decision to choose two of three of the following materials to put inside a cup with a hole in the bottom: cotton balls, gravel, and coffee filters. They used two of these materials and built a filter around the cups that were premade for them. Dirty water was filtered through, and an RCX attached to a light and a light sensor was used to measure how much of the light passed through (turbidity).