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This week, we had a lesson about environmental engineering and filters. First, we discussed what an environmental engineer does--more than just someone who "helps the environment," environmental engineers use engineering principles to improve the environment through projects such as waste management and pollution control. Next, we introduced the lesson, and explained how different materials in a filter trap the large dirt particles while the small water molecules can get through. We gave the students a variety of materials (gravel, sand, cotton balls, paper, etc.) and set them loose to build the best filter possible. A few groups succeeded in cleaning the very dirty water, but most groups were still confused about what made a good filter. So we debriefed the activity with a metaphor using doors--the more porous materials should go at the top, as they are like large doors that stop the biggest dirt particles from getting through, and the least porous materials such as sand should go at the bottom, as they trap the smallest particles.