We remember doing this one as kids! Use a sharp pencil to poke a hole in the bottom of 2 paper cups. Thread through 5m of string or yarn and tie a knot. If it keeps coming through the hole, tie a paperclip to the end so it wont go through. Be sure you stand apart with the string pulled tight as the vibrations from when you speak into the cup, travel through the string and into the other cup! But remember to keep the string tight!