Build and Test Your Filter

Follow these steps to build your filter:

Step 1: Take the top half of a plastic bottle and turn it upside down like a funnel.

Step 2: Place a piece of cloth or coffee filter over the mouth opening.

Step 3: Add a layer of cotton balls (about 2 cm thick).

Step 4: Add a layer of fine sand (about 5 cm thick).

Step 5: Add a layer of small gravel (about 3 cm thick).

The order matters! Water flows down through the layers. The gravel catches big stuff first, the sand catches medium stuff next, and the cotton catches the finest particles last. Each layer does a different job!