When one person talks into their cup, the cup vibrates. the sound particles of these vibrations passes along the string. the string is a solid, so the particles are very close together, and the vibration can pass quickly and easily along the string. the vibrations pass from the string into the second cup which also vibrates. these vibrations pass through the air particles into the second persons ear who can then hear the sound of the first persons voice. the sound of the first persons voice is energy through the air over the same distance.
