The child speaking into a cup creates sound waves which cause the bottom of the cup to vibrate.
Since the string is tightly attached to the bottom of the cup, the vibrations of the cup are transmitted
to longitudinal waves in the string. These longitudinal waves travel down the string, and cause the
bottom of the receiver cup to vibrate. This relatively large vibrating surface moves the adjacent air,
and generates sound waves from the bottom of the cup, traveling up into the cup. These waves are
incident on the receiver’s ear, and they hear the sound from the speaker.