Just poke a whole in the bottom of the cups, string the string through, and tie the paper clips to the ends of the string so the string doesn't fall out. SUPER easy!
This is our short version that we used in the classroom. We had to whisper to make sure no one could hear except the person on the other end. It was actually easier to just tap on the cup or make quiet noises, instead of speaking since we were soooo close together.
We also made one MUCH longer that we took outside to try. Sorry, didn't take a pic. We noticed it was really important to pull the string tight when speaking.
And here is the worksheet we used during the investigation. I found this somewhere online awhile ago, but I don't remember where. Please let me know if you know where this is from! I would love to give the person credit. You can get it here.
We are continuing the investigation today with different kinds of cups so that's why we didn't finish filling out the back of the page. Enjoy!