This video shows how to make a telephone using two tin cans and a piece of string. Experiment using a much longer string than shown and speaking into the cans in a low voice that the person at the other end cannot be hear through the air. I have been amazed at how effectively this device transmits sound. Have kids discuss how it works in terms of transmitting vibrations. Why does the string between phones need to be kept tight. Why must the string not touch anything between the phones.