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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.
From the LAB
What you will need
Two paper or plastic cups — tin cans work even better if available (stronger resonance)
String — approximately 5–10 metres of cotton or nylon to start
A sharp pencil or skewer to make a small hole in the base of each cup
Two small paperclips or a piece of tape to anchor the string inside each cup
Optional for variable testing: nylon fishing line, wool, and different cup sizes or materials
How to do it
1
Build your phone
Use a sharp pencil or skewer to make a small hole in the centre of the base of each cup — just large enough for the string to pass through.
Thread one end of the string through the hole in the first cup, pull it inside, and secure it with a paperclip or a knot so it can't pull back through.
3
Test the phone
Both students hold their cups and walk apart until the string is completely taut — no sag at all.
One student speaks quietly directly into their cup (not shouting — a normal speaking voice or even a whisper).
The other student holds their cup to their ear and listens. Swap roles.
Then try whispering.
5
Test some variables
Now test at least two variables, keeping everything else the same.
Try: a different string material (fishing line vs wool vs cotton), a different string length, or a different cup type (tin can vs paper cup vs plastic cup).
2
Make your prediction
Before testing: hold both cups with the string hanging loose between them.
Predict — will the telephone work with the string slack? Then predict what will change when the string is pulled taut.
4
Test the slack
While your partner continues speaking, deliberately let the string go slack — allow it to sag, or gather a loop in your hand so there is no tension.
Listen to what happens.
Then pull it taut again. Repeat several times.
Did it work? Share the science! Tag @the_crazy_scientist on Instagram — we love seeing your experiments!
The Whisper Wire
Designed by Darin Carr (BSc, DipEd)
NESA Accredited Teacher Chemistry & Physics Specialist
Creator of the LAB™ Learning System
Before electricity, before radio, before the internet — two cups and a piece of string were already transmitting voices across a room. Most people accept that it works. Fewer ask why a slack string ruins it instantly.
5-12 yrs
Easy
20
min
Stage 1-3
>
The Whisper Wire
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.
You've already discovered that sound travels through solids more efficiently than through air — a coat hanger proved that. Now the question is: does ANY solid transmit sound, or does the solid need to be in a specific condition for it to work?
You have two cups and a piece of string. Before you build anything or test anything: predict — if the string between the cups is pulled tight, do you think the voice will carry clearly?
ecall what you heard through the cup when the string was taut — and then when it went slack. Describe the difference as precisely as you can. What changed, and how quickly did the change happen?
Think about what you felt when you touched the string while your partner was speaking into their cup. What was the string doing — and what does that tell you about what was travelling through it?
You already know sound travels well through solids. But the slack string test showed that simply being solid isn't enough — something else needs to be present.
What is the difference between a taut string and a slack string that makes one carry sound and the other not?
You've just built a working communication system from two cups and a piece of string.
The same principle — tension carrying vibration through a solid medium — is behind every communication technology that came before wireless signals. Think about where it's still at work.
A spider doesn't have ears. It detects prey, predators, and potential mates entirely by feeling vibrations in its web. The silk threads of a web are always under tension.
Based on your string telephone, why does the tension in the web matter — and what would happen to a spider's ability to sense vibrations if the web went slack?
"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.
Does the string material change how well the voice carries — does nylon fishing line transmit more clearly than cotton string or wool?
Does the cup material change the sound quality — does a tin can produce a louder or clearer sound than a paper or plastic cup?
🧪 Try it! Change ONE thing and test again. What did you discover?
Want to go deeper? Tap a section below to explore. ▼
The Science Behind It
The string telephone seems simple — but the reason it works, and the reason it instantly stops working when the string goes slack, reveals something fundamental about how waves travel.
When you speak into the cup, your voice makes the air inside vibrate. Those vibrations reach the base of the cup and set it oscillating. The cup base is in contact with the string — so the vibration passes into the string. Now here is the critical part: for the string to carry that vibration to the other end, it must be under tension.
A taut string behaves like a stretched spring. When one section of it is displaced — pushed forward by the vibration — the tension in the string acts as a restoring force, pulling it back. This creates a wave: a series of compressions and expansions travelling along the string from one cup to the other. The tighter the string, the more efficiently the wave travels.
A slack string has no restoring force. When you displace one section, it just flops — there is nothing to propagate the disturbance to the next section. The vibration dies immediately. That is why one centimetre of slack destroys the entire telephone.
At the receiving end, the string vibrations reach the base of the second cup and set it oscillating. The cup then acts as a resonating chamber — its shape and material amplify the vibration back into audible sound for the listener's ear. This is why tin cans produce a clearer, louder result than paper cups: metal resonates more efficiently than paper, which absorbs more of the vibration energy (as you discovered in The Spoon Orchestra).
Extension: G&T Years 5 & 6
Vocabulary
Know a parent or teacher who'd love this? Send it on! 👇
The Crazy Scientist books
These highly visual books combine storytelling and real science, helping students revisit key concepts and stay engaged long after the session.
Designed by a practising NSW classroom teacher (30+ years experience), these books directly support NSW Science & Technology (2024) outcomes and reinforce “Working Scientifically” skills.
Perfect for classroom libraries or home explorations.
For teachers (YouTube)
— Science Before the Bell
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