Tom and Zoe make a string telephone. They use two empty tin cans joined by a piece of string. Tom and Zoe are 5 m apart. Tom talks quietly into one tin can, and Zoe holds the other tin can to her ear. If they keep the string taut, the sound of Tom's voice travels along the string. How does the sound travel along the string to Zoe?

Sound is a vibratory disturbance in a medium which carries energy from one point to another. The particles vibrates and the energy get transferred to next particle and from that to the second next particle and so on. In this way the sound travels from one point to another in a medium.
 
String has a higher density than air, this help the sound waves move over a greater distance.
Voice vibrated the air inside of the cup, which in turn made the bottom of the cup vibrate. These vibrations transferred to the string and then into the bottom of Zoe's cup, which made the air inside cup vibrate and sound become detectable. When the string goes slack, the vibrations dissipate more easily and get lost along the way. 

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