Cut a soda bottle in half. Mix water with oil, food color dye, or even pieces of Styrofoam and pour the solution into the bottom half of the soda bottle. Turn the soda bottle upside down and add a cotton ball to the inside of the bottle's neck. Pour water from the bottom half of the bottle through the top half of the bottle, and out through the neck into a container. Examine how clear the water is after it filters through the cotton ball. Add more cotton balls until they begin to work more effectively. Keep track of the number of cotton balls you need to add to the inside of the bottle's neck until the water is clear.
Use the soda bottle from the first project. Mix some water with dirt, then create a filter from a paper napkin and sand. Add a napkin to the inside of the neck of the soda bottle, then add a little sand on top of the napkin. Pour the water through the neck of the bottle and record how well the filter works. Increase the amount of sand, keeping track of the amount of the sand you need to add before the filter of paper napkin and sand is effective.
Create a water filter by using a combination of materials. Line the inside of the neck of the bottle with a paper napkin, then add some cotton balls then some sand on top of the cotton balls. Pour the water through the filter you have made to study the results. Experiment with the materials by changing the amounts that you use and record how well the amounts of each material works the best.