How many drops fit on a coin?
You will count the water drops that sit on one coin. The pile can rise above the edge, but a badly placed drop may spill it before the pile becomes large.
- Science
- Starter
- About 10 minutes
Grown-up needed: yes for setup, nearby supervision and handling water or small items
What you need
- A coin selected by a grown-up
- A cup of clean water
- A dropper or a drinking straw
- A tray
- A towel
The rule
Count every drop.
How to do it
- Ask a grown-up to set the coin flat inside the tray.
- Predict how many drops will stay on it.
- Place one small drop near its centre.
- Continue adding drops from the same low height.
- Stop at the first spill and record the previous total.
- Dry the coin and repeat with similar drops.
Why it works
Water molecules attract one another. At the surface, those attractions produce surface tension, which helps the water keep a rounded outline. The coin gives the growing puddle a place to sit, and its edge can help pin that outline. Drop size, dirt and the coin’s angle affect the spill point. A high drop can disturb the pile before it is full, so your count only compares trials well when you add drops in a similar way.
Make it harder
- Try a larger coin with the same dropper.
- Compare a low drop with a slightly higher one.
- Make repeated counts close together by controlling drop size.
Ask Taz
- Why does the water rise above the coin?
- Why can one falling drop spill the whole pile?
- Why does a tilted coin hold less water?
For grown-ups
A grown-up should stay with the coin and prevent younger children from handling it. Do not drink the test water. Counting supports careful recording, but matching drop size is just as useful. Ask at dinner: “What would you change if your three counts were very different from one another?”
Taz is an AI and can be wrong, so stay close by. Read how safety works on Taz.
Show us yours
Film your hands and what you made, not your face or name. Ask a grown-up before posting anywhere else.
Questions parents ask
Can we use what we already have?
Start with the listed household materials. Keep the basic setup small and ask a grown-up to choose suitable items before beginning. The stated time is an estimate, so let the observation decide whether you need longer.
What should we record?
Note the prediction, the result and the one thing you changed. Keeping those separate helps a family discuss what happened without turning an activity into a competition.
Must the result be shared online?
No. A conversation at home is enough. If you choose to share, keep identifying details out and follow the grown-up permission line above.
More to try
See all Taz Challenges, or look through the science labs. One place for every why. Ask for early access at aitaz.com