Things you need:
Please do this and all experiments with adult supervision. First, thread the needle with the thread. Next, use the needle to pierce the center of the bottom of one of the cups and pull the thread through. Now, please remove the needle carefully and set it aside. Next, on the inside of the cup, tie the thread around a toothpick or paperclip to make a knot. This will prevent the string from being pulled through the cup. If you use a toothpick, you may have to break it in order for it to fit the bottom of the cup. Repeat the above steps for the other end of the string with the other cup and you will have 2 connected telephones!
To use the telephones, you and a friend will need to each hold a cup and slowly walk away from each other, gradually pulling the line taut. Be careful not to pull the string out of the cups! Make sure the string is tight and it is not touching anything. Now one person will talk into their cup "phone" and the other will hold their cup over their ear. Can you hear what the other person is saying? Now try switching!
How does it work?
Sounds vibrations travel most easily through solid objects. The bottoms of the cups act as diaphragms that both transmit and receive the vibrations of the thread. When you talk into the cup phone, the sound waves from your mouth cause the bottom of the cup to vibrate. These vibrations are then transmitted along the string to the bottom of your friend's cup making it vibrate as well. This vibration then creates sound waves that travel right into your friend's ear.
Can you think of other materials we can use to make a phone?
Will a longer string make it easier or harder to hear?
What happens to the sound if the string touches something?