An electroscope is a device that indicates the presence of static charges. You can use balloons to make an electroscope and witness the effect of static charges before your very eyes!
Note: This activity works best on a cool, dry, low-humidity day.
What we call electricity is really a collection of tiny particles called electrons. If electrons are in motion, we call them current electricity. If the electrons are not moving, we call them static electricity.
The stroking motion removes a number of electrons from the cloth (which acts as a charging cloth) and deposits them on the balloon, creating a static charge. Rubber is not a good conductor of electrons, so the electrons remain on the surface of the balloon.
Electrons bear a negative charge, and objects with like electrical charges repel—which is why the balloons push away from each other. Objects with unlike electrical charges attract each other. You might notice that the charging cloth often is attracted to the balloons.