You will need:
Two tin cans (or paper cups, or plastic cups)
One piece of string or thin rope that is long enough to connect both the “phones”
Poke holes in the bottom of the cups – the holes should be about the size of the string that you are planning to use.. Run the string through the hole in the bottom of one of the cups and, on the end of the string that is on the inside of the cup, tie a knot so that the string won’t pull back through. Repeat this on the other cup with the other end of the piece of string. You now have two cups (or tin cans) that are connected by a piece of string. If you pull the cups so that the string is taught, you can hear in one can what is being said in the other can. The sound passes down the string from one person’s voice on one end to the other person’s ear at the other.
The advantage to using a tin can is you can tap it and the sound goes along the string. This is VERY handy for doing Morse Code or for getting the attention of the person at the other end!