Natural Water Filtration Experiment

A large plastic bottle (2-liter soda bottle works perfectly)

Scissors for cutting the bottle

Cotton balls

Activated charcoal (available at pet stores — aquarium charcoal)

Fine sand (play sand or beach sand)

Coarse gravel or small pebbles

A sample of muddy water

Clear jars for comparing before and after

Cut the bottom off a large plastic bottle. Turn it upside down so the cap end points down. Remove the cap. Place the inverted bottle in a jar or cup — the filtered water will drip into it.

Build the filter layers from bottom to top. First, stuff a cotton ball loosely into the bottle neck (this keeps everything from falling through). Then add about 2 inches of activated charcoal. Then 2-3 inches of fine sand. Then 2 inches of coarse gravel on top.

Make your test water by mixing garden soil with water in a separate container. Make it really muddy — the worse it looks, the more dramatic the filtration.

Pour muddy water slowly through the top of your filter. Watch it seep through each layer.

Observe the water dripping from the bottom. Hold a clear jar of muddy water next to the filtered output. The difference should be dramatic — the filtered water will be much clearer.

Now iterate. Build a second filter with different layer thicknesses. Which design produces clearer water? What happens with no charcoal layer?

Important: filtered water from this experiment is NOT safe to drink. It removes particles and some contaminants, but not bacteria or chemicals. This is a demonstration, not a purification system.

The charcoal layer is the key. Activated charcoal adsorbs dissolved organic compounds that sand alone can't catch. Skip it and compare — the difference is visible.

This is how natural groundwater filtration works. Rain percolates through soil, sand, gravel, and rock — nature's filter cleans it on the way to the aquifer.

Municipal water treatment plants use the same principle at enormous scale. Your child just built a miniature version of essential infrastructure.