This activity adapted from Exploratorium Cuica activity: www.exploratorium.edu/video/cuica-activity-introduction.
Prepare the cups for the students:
Use the straightened paper clip to make a hole in the bottom of each cup. The hole should be easily wide enough to thread the string through. If the cups are plastic, heat up the paper clip to push it through the plastic without breaking it.
Give each student a cup with a hole in it, a length of string, a paperclip.
Demonstrate how to thread the string through the hole into the cup, then knot the end of the string around the paperclip to hold the string in the cup. If necessary, use large rope to demonstrate how to tie the knot.
The students hold the cup in one hand and run their fingers of the other hand down the string, pinching the string between their thumb and pointer finger.
They can first try with a dry string, to make sure they all have the motion correct, and to contrast the (lack of) sound made with the next step.
Then they dip their fingers in water, before running them down the string again. A loud squawking sound is made.
Students can put a small rock in their cup and watch it as they make the sound. The rock should bounce around, maybe giving students a clue as to how the sound is made.
The sound is made as the wet fingers slip and grip their way along the string, making it vibrate. This vibration is transferred to the cup (as seen by the bouncing rock). The vibrating cup makes the air inside it vibrate. The vibration transmits through the air to your ear, where it vibrates the eardrum and we sense the sound.
Cups of different sizes make different sounds: the smaller cups make a higher note.
Cups of the same size made from different materials do not make noticeably different sounds.