What children will learn
- Understand how layers of different-sized materials trap particles
- Run a fair test and record observations
- Learn why filtering alone does not make water safe to drink
You'll need
- A large clean plastic bottle (an adult cuts it in half)
- Cotton wool balls or a coffee filter
- Clean sand (about 1 cup)
- Small gravel or pebbles (about 1 cup)
- Garden soil and water to make muddy water
- Two clear jars or cups
- A timer and a pencil
Safety first
- The filtered water is NOT safe to drink. Filtering removes visible dirt but not germs or dissolved chemicals.
- An adult should do all cutting; the cut plastic edge can be sharp.
- Wash hands with soap after handling soil and muddy water.
- Pour used water onto the garden, not down the sink, so sand and soil don't block the drain.
How to do it
- An adult cuts the bottle in half. Turn the top half upside down and sit it in the bottom half like a funnel, with the lid removed.
- Build the layers from the neck up: a plug of cotton wool, then sand, then gravel on top. Each layer should be about 3–4 cm deep.
- Mix a cup of muddy water in a jar. Pour a little into a second jar and keep it as your 'before' sample.
- Slowly pour the rest through the filter. Time how long it takes to drip through and compare the 'after' water with the 'before' sample.
- Rate the clarity from 1 (very muddy) to 5 (very clear) and record it on the sheet.
- Change one thing — swap the layer order, add more sand or filter the water a second time — and test again. Which design worked best, and why?
Make it easier
Build one filter together and simply compare the 'before' and 'after' jars.
Make it harder
Test three designs fairly using the same amount of muddy water each time, then research the extra steps water treatment plants use to make water safe.
Parent tip: Ask 'It looks clear — so is it safe?' The answer (no!) leads to a great chat about germs and why our tap water goes through careful treatment.
Water Filter Test Results
Name: ____________________ Date: ____________
| Design | Drip time | Clarity (1–5) | Notes |
| Test 1 | |||
| Test 2 | |||
| Test 3 |
Change only one thing between tests. Remember: filtered water is not safe to drink.
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Fantastic work! Every little step helps you grow.