Year 4 investigated and tested string telephones.

A string telephone is a simple, two-cup device that transmits sound using vibrations along a taut string. When a person speaks into one cup, the sound waves vibrate the bottom of that cup, which in turn causes the string to vibrate. These vibrations travel along the string and into the second cup, making the air inside it vibrate and producing sound waves that can be heard by the person listening at the other end.