Although there are many different types of string available to the tin-can telephone engineer, a thick jute-like string is best for carrying messages over 30 feet or more. Yarn is best for any distances below that, states National Partnerships for After School Science.
Children have made telephone lines out of tin cans and string for decades, says NPASS2. It's best to avoid thin strings such as thread, because it is likely to break under the tensile strength needed to make an appropriate connection between a transmitter can and a receiver can, the agency says. A thread would likely work much more effectively if the tin cans were substituted by small paper cups, such as those used at water dispenser machines. However, should tin cans be used, a heavy-duty string is definitely a wise choice.