The bottom of the first cup vibrates the string, which in turn vibrates the bottom of the second cup.
Work Step by Step
The first child’s vocal cords create sound waves that vibrate the bottom of her cup. The string is stretched tautly between the 2 cup bottoms, and it transmits the vibrations of the first cup to the bottom of the receiver’s cup. This large vibrating surface, much like a music speaker, moves the air in the cup and creates sound waves that enter the other child’s ear.