Final answer:
The simplest tie for creating a party line in a science experiment would be a slip knot or a square knot. These knots are easy to tie and untie, facilitating three-way communication between cans or cups connected by a string.
Step-by-step explanation:
The question revolves around the concept of creating a party line using cans or cups and a string, which is a simple way to demonstrate the transmission of sound through a medium in a science experiment. To tie a third string to the middle of an existing two-way string communication system, you can use a simple knot that is easy to tie and untie such as a slip knot or a square knot. Once tied, this setup allows three people to communicate with each other, as sound travels from the cans or cups attached to one end of the string to another, though it may be more difficult to maintain the tension required for clear sound transmission.