The Water Magnet
Running water has a secret: point a stream at a curved surface and the surface wins. This experiment reveals one of physics' most surprising tricks — invisible air pressure can make a ping pong ball chase a stream of water across a sink.
5-12 yrs
Easy
10
min
Stage 1-3
Mission Briefing.
Designed by Darin Carr (BSc, DipEd)
NESA Accredited Teacher Chemistry & Physics Specialist
Creator of the LAB™ Learning System
Mackey
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The Water Magnet
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NESA Accredited Teacher
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High school chemistry & physics specialist 30+ years
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The Crazy Scientist in primary schools — 15 years
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International conference presenter on science education
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Creator of the LAB™ Learning System
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Curriculum aligned: NSW Science & Technology K–6 (2024)
A picture is worth a thousand words — check this out and see if you can spot the science hiding in plain sight.
Mission Equipment
1 ping pong ball
1 metal spoon
Tape
A tap or faucet with running water
A sink (or large bowl to catch water)
A piece of string (~30 cm) to hang the ball
Let’s Investigate
1
Start with a spoon
Turn your tap on to a thin, steady stream — about the width of a pencil.
Hold the back of a spoon (the curved side) close to the stream without letting them touch.
Watch what the water does when it reaches the spoon's curved surface. Does it bounce away — or does something else happen?
3
Place near water stream
Hold it in the air close to, but NOT touching, the running stream. Before you move it any closer: what do you predict will happen?
5
Observe
Once the ball is right beside the stream, relax your fingers a little.
Notice whether the ball tries to move on its own — and what it feels like in your fingers. What is holding it there?
Notice whether the ball tries to move on its own — and what it feels like in your fingers. What is holding it there?
2
Make your ball on a string
Tape a ping-pong ball to a 30 cm length of string.
4
The test
With the ball about 3–4 cm from the stream, slowly slide it sideways toward the water.
Watch carefully — something unexpected should happen before the ball even touches the stream.
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The Crazy Scientist LAB Learning System™
Every experiment follows The Crazy Scientist Lab Learning System™ — a simple way to help kids think like real scientists.
We
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LINK to what they already know,
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ACTIVATE curiosity through hands-on discovery
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BUILD understanding that actually sticks.
Think about a magnet pulling a paperclip toward it. Now think about water running from a tap — it flows, it splashes, it soaks things.
Water can't pull a solid object toward it from a distance. That's not how water works.
Or is it?
One curved surface is about to change your mind.
Predict: what will happen when you hold a ping pong ball next to a thin stream of running water — without letting them touch? Write it down before you try.
You have run the experiment. Think back to the moment the ball got close to the stream —describe exactly what happened.
Compare what you felt in your fingers in Step 4 and what you saw in Step 5. Were they telling you the same thing?
When the ball was next to the stream, did it feel like it was being pulled by the water, pushed by something else, or held in place by both?
When you used the string, the ball leaned toward the stream — but the stream was hitting one side of the ball. What was pushing from the other side?
Fluids — water and air both — cling to curved surfaces. Scientists call this the Coanda Effect.
The curved upper surface of an aircraft wing forces air to speed up and cling — this creates lower pressure above the wing than below. How does that lift a plane weighing hundreds of tonnes?
"Want the full teacher guide? The Crazy Scientist Lab includes classroom delivery tips, how to manage the WOW moment, differentiation for Stage 2 & 3, — ready to teach tomorrow."
Think Like a Scientist
Scientists don't just do ONE experiment; they change one part of the experiment (independent variable) and then see how it affects another part of the experiment
(dependent variable)
Change ONE variable and test again.
What happens if you replace the ping pong ball with a heavier ball — does the Coanda Effect still work?
Does the thickness of the water stream change how strongly the ball is pulled toward it?
🧪 Try it! Change ONE thing and test again. What did you discover?
Dr Puddledrip’s Science Tip
Want to go deeper? Tap a section below to explore. ▼
The Science Behind It
Why does the ball move toward the water?
When the stream of water touches the curved surface of the ball, it clings and wraps around it instead of bouncing away.
Moving fluid clings to a curved surface and follows its shape — this is called the Coanda Effect.
As the water wraps around the ball's curve, it creates a region of lower pressure.
The normal air pressure on the opposite side is now relatively higher. It pushes the ball toward the water, not the water pulling the ball, but air pushing it.
What looks like a magnetic attraction is actually a pressure difference caused by a curved surface and clinging fluid.
Why water clings to a curved surface?
Water molecules are attracted to each other and also to many solid surfaces. When water flows past a curve, adhesion (attraction between two different surfaces) between the water and the surface keeps the water in contact rather than flying off in a straight line.
Why does the string angle prove a force is acting?
When you hang the ball on a string above the tap stream, the string doesn't hang straight down — it leans toward the water.
The only thing that can make a hanging string lean is a horizontal (sideways) force. The string angle is direct evidence that something is pushing horizontally.
Real-world connection
Tap water running down the outside of a spout instead of falling freely is the Coanda Effect in everyday life — the water clings to the curved surface rather than detaching.
Aircraft designers use the Coanda Effect to deflect thrust, control lift, and reduce drag — shaping engine exhausts and wing surfaces so that airflow clings where it's needed.
Try next
For another experiment where moving air creates a completely counterintuitive pressure result, try [The Impossible Blow] — where blowing harder keeps a ball trapped rather than pushing it away. And if you want to see the same Coanda logic applied to a pair of balloons, [The Stubborn Balloons] will produce a result that almost nobody predicts correctly the first time.
Extension: G&T Years 5 & 6
Why water clings instead of flying off When water flows past a curved surface, two forces compete:
Inertia, which tries to make the water fly straight
Adhesion, which keeps the water stuck to the surface
Water molecules are strongly attracted to many solids. As the stream curves around the ball, adhesion wins — the water bends instead of breaking away. This bending is the start of the Coanda Effect.
Question: If you coated the ball in oil (which water does not stick to), would the effect be stronger, weaker, or disappear?
Vocabulary
Coanda Effect: The tendency of a moving fluid to cling to a curved surface and follow its shape.
Adhesion: The attraction between water molecules and a solid surface, helping water stick instead of flying off. (two different surfaces)
Pressure difference: A difference in pressure between two regions. Higher pressure on one side pushes the ball toward the lower‑pressure side.
Equilibrium: A state where all forces on an object balance. The string angle shows the ball is not in equilibrium — a sideways force is acting. (balanced forces)
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READY TO TEACH THIS
TOMORROW?
Running the experiment is easy; however, teaching it well is another challenge.
Teachers often ask:
How do I adapt this for Stages 1,2 or 3?
What do I do with fast finishers?
What misconceptions will they have?
How do I structure this for a full class?
What syllabus outcomes does it cover?
What do I say when they ask WHY?
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