This experiment is great for understanding water filtration. We use 2 liter bottles with the bottom cut off. Students preplan in their groups what materials to use and how to place them in the upsidedown 2 liter. Once the prepare their "filtration system", I pour dirty water in and we collect it (glass baby food jars work great because you can see inside) out of the small opening at the bottom. Students then evaluate what they could improve, empty the 2 liter and try again. We compare all of the