Paper cup telephone

Mobile phones had not been invented when the Grandma in this story, Jean, was born, and it was only the very privileged that had any sort of telephone - which in those days was a chunky looking land-line phone.
Here are some instructions to make an easy telephone to whisper secrets and stories to your friend:  

You will need: Two paper cups, one piece of string (long enough for you to be out of hearing range of your friend) and something to pierce a hole in the cup.

Pierce a small hole in the cup - the smaller the better, but it needs to be big enough to pass the string through.

Pass the string from the bottom to the inside of the cup and pull some through the mouth of the cup.

Tie a large knot in the end of the string (I used a slip knot)

Pull string back through the cup so that the knot rests on the bottom of the cup (effectively blocking the hole)

Follow the same method with the other cup, using the other end of the same length of string, so that you have two cups joined by string.

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To use, you hold one cup, and your friend holds the other. Walk away from each other as far as the string allows. Make sure string is taught (but not too tight as the knot could slip through the bottom of the cup - if this happens, some sticky tape might help) and you cover your mouth with the cup and speak into it, while your friend holds his end up to his ear. You speak and he listens. Then reverse - you hold you cup to your ear and listen as your friend speaks into their cup. 
Experiment - does it work as well if the string is loose; does it work as well if something is touching the string (eg a finger/a wall); does it work around corners; how long can you make the string and it still work?