In this problem, students model the charges in a balloon before and after it is rubbed with a wool sweater. They are told the balloon has a negative charge after rubbing. We anticipate a range of ideas about charges: (1) polarization instead of a net charge; (2) positive charges as the kind that "move;"  (3) charges as "activated" or given energy to attract/repel; (4) charges as created or donated by the sweater and not otherwise present.

Here a student uses an "activation" idea. The diagram is less clear, but the writing suggests that the charges are there but not always "charged." 

"Polarization" but not in a traditional sense. This suggests the balloon is like an atom: with the negative charges on the outside, there may seem to be a negative charge even though the overall charge is neutral.