Let's build a telephone that needs no batteries at all! Sound is a shake, and a tight string can carry the shake from one cup to the other. This easy build is from the Make Some Noise shelf of the Peabody Workshop (shakes in the air you can hear). It fits grades K–3rd, takes about 15 minutes and costs nothing if you have the things below at home. One step is a grown-up's, and the bench marks each one.
What you need
Paper cups: 2
String or yarn: a long piece, as long as a room
Paper clips: 2
A pencil
Tools: Kid scissors.
Safety
Small parts! Keep them away from little brothers and sisters.
Steps
A grown-up does this one. A grown-up pokes a small hole in the bottom of each cup with a pencil. Push the pencil from the inside out, slowly. The hole only needs to fit the string.
Cut a long piece of string, as long as your room. Longer is fun, but the string has to stay tight, so pick a length you can stretch out.
Push one end of the string through the hole, into the cup. Twist the end to a point, or wrap a little tape round it, so it slides in.
Tie the end inside to a paper clip, so it can't slip out. The paper clip is bigger than the hole, so it holds the string when you pull.
Do the same with the other cup on the other end.
Walk apart with a friend until the string is tight and straight. Don't let the string touch anything: a chair or a wall soaks up the sound.
Talk into your cup while your friend holds theirs to an ear.
Test it
How many big steps apart can you still hear a whisper? Count in steps: more is better. Try it twice and keep the better number.
Why it works
Your voice shakes the bottom of the cup. The tight string shakes too, all the way along, and shakes the bottom of the other cup. That cup shakes the air inside it, and your friend's ear hears the shake as your voice. A floppy string can't pass the shake along.
Make it better
Try a longer string, or a fishing line.
Let the string go floppy: can you still hear?
Add a third cup tied to the middle of the string for a party line.