Have each girl assemble a static tester. First take the striped twist tie and feed it through the hole in the bottle cap until it wedges in. If wire does not wedge in, use tape. Bend the top of the wire (the exposed wire on the outside of the bottle cap) like a hair pin. Bend the other bare end like a fishing hook. Take a piece of aluminum foil and hang it over the fishing hook. Gently replace the whole fixture back into the bottle allowing the wire to hang freely. Take a balloon and rub it on your hair until a significant static charge builds up. Hold the balloon next to the “hairpin” wire without touching it. Watch what happens to the Aluminum strip.

Explanation:

When you rub the balloon against your hair, the balloon takes negatively charged particles, called electrons, from your hair. Then both the balloon and your hair have a small charge of static electricity. Your hair has a positive charge, and the balloon has a negative charge. Since any two objects with the same charge push away from each other, the balloon pushes away anything that also has a negative charge, including electrons.
That pushing makes the electroscope work. When the balloon comes close to the wire, the balloon negative charge pushes some of the wire’s electrons down into the foil strip. Some electrons go into one side of the foil, and others go into the other side. Now both foil flaps have a negative charge, and they push away from each other. The stronger the charge, the wider they open