People need water to survive and yet 1.1 billion people in the world do not have access to safe drinking water. Finding ways to clean water is important as dirty water can cause health problems. Filtration is a cleaning method that is used by water companies worldwide and is also a natural process that occurs when water seeps through materials, such as sand and rocks, in the ground. While you can use sand and rocks to create your own water filter, you'll need some other materials to help.
Use the knife to cut 10 cm (4 inches) off the bottom of one water bottle.
Remove the bottle cap. Attach the coffee filter to the outside of the bottle neck using the elastic band.
Cut 10 cm (4 inches) off the top of the second water bottle.
Turn the first bottle (the one with the coffee filter on it) upside down and insert it, neck first, into the second bottle. The top bottle is the filter and lower bottle is the water collector.
Pour the gravel into the bottle opening. The coffee filter should stop it falling through the bottle neck. Add the coarse sand on top of the gravel. Tip the fine sand on top of the coarse sand to create a layered effect.
Pour the tap water through the filter to clean it.
Pour the dirty water into the filter to test it. The water collected in the lower bottle should be filtered clear. Your filter will now be complete.
Trim the plastic bottles if they do not fit snugly into each other. If preferred, a beaker can be used as a water collector. When cleaning the filter, pour the water from a low height so that it does not disturb the sand. For the fine sand use beach or play sand and for the rocks use natural colour aquarium rocks. To make even cleaner water add a layer of carbon on top of the fine sand.
Do not drink the water. The filter is for experimental purposes only. Be careful when cutting the bottles, and use adult supervision if necessary. Dirty water can contain bacteria. Wash your hands with soap after handling it.