String Telephone
Before radio, before wires, there was string. Two cups and a piece of thread carry real conversations — and reveal exactly how sound travels through solids.
Use child-safe scissors for poking holes. Keep the string away from necks — never loop it around anyone.
Materials
- Two paper or plastic cups
- 10 to 20 meters of string or thread
- Two paperclips or small buttons
- A pin or sharp pencil
- A friend
Procedure
Build the phones
Poke a small hole in the bottom of each cup. Thread the string through, tie it to a paperclip inside each cup so it cannot pull out.
Pull taut and talk
Walk apart until the string is straight and tight. One person talks normally into their cup while the other holds their cup over one ear. Swap roles.
Go slack
Let the string sag in the middle and try talking again. Note how the sound changes.
Touch the string
While your friend talks, lightly pinch the string in the middle. Then pinch it hard. Describe what happens to the sound.
Test different lines
If you have thread, yarn, and string, swap them in. Which material carries the voice clearest?
What to observe
With a taut string, voices come through clearly — even whispers.
A slack string kills the sound almost completely.
Pinching the string hard mutes it; a light touch barely matters.
Why it happens
Your voice vibrates the cup bottom, launching a mechanical wave down the string. A taut string transmits transverse vibrations efficiently because tension restores each segment quickly; a slack string absorbs the motion as floppy, lossy wiggling instead. Pinching hard creates a node that reflects the wave, while a light touch barely disturbs it — the same reason guitar strings must be tight to ring and frets change their pitch.
Try variations
Try a three-way call by knotting a third cup into the middle of the string.
Compare whispering versus talking — which survives the string better?
Replace the string with a wire coat hanger (carefully) and compare loudness.