In this activity, learners simulate nature's water filtration system by devising a system that will filter out both visible and invisible pollutants from water. Each group is asked to design a water filtration system given only a limited number of materials. They are asked to both sketch and write down step by step procedures before filtering a sample of the "dirty" water. Learners make modifications and share their findings about what worked or didn't work. This activity is the second of a set of activities on the page and can be used independently or in conjunction with the first activity called "Who Dirtied the Water?" in which learners simulate the sources of water pollution.