Gather two empty cans, string, and a sharp tool for making holes
Make a small hole in the bottom center of each can
Thread one end of the string through one can from the outside to the inside
Tie a large knot inside the can so the string cannot pull back through
Repeat with the other end of the string and the second can
Pull the string tight between the cans
Test the phone by speaking into one can while a person listens at the other end