Forgive me for taking the so-called 'scenic route' to your answer, but I believe that this example may be more useful to your student than simply being told which one it is.
There is a common experiment used in many courses on electricity which involves rubbing two plastic rods with fur. When the two rods are brought close to each other, they actually repel from each other, showing that both rods have acquired the same charge. The same is true when two glass rods are rubbed with silk and brought together. However, when a plastic rod rubbed with fur is brought close to a glass rod rubbed with silk, they attract.
This phenomenon was first noticed by Benjamin Franklin, who put forth the idea that "every object has a 'normal' amount of electricity that can be transferred from one object to the other when two objects are in close contact, as when they are rubbed together." (Physics for Scientists and Engineers - Volume 2
, by Paul A. Tipler) Ben Franklin chose to call these charges 'positive' and 'negative', rather arbitrarily saying that the glass rod had the positive charge. Since the plastic rod is attracted to the glass one, the plastic rod must have the negative charge.
Since then, we have learned that the 'negative charge' that he described is actually the result of an object acquiring additional electrons. Likewise, the 'positive charge' occurs as the result of the loss of electrons. The plastic rod being rubbed with fur is very much like the plastic comb you asked about. Like the rod with fur, when the comb is pulled through hair, it acquires a negative charge. That is, it picks up more electrons than it had before.
Another example of this the common trick of rubbing a plastic balloon in someone's hair. After being rubbed together, the plastic in the balloon picks up a negative charge, while the person's hair ends up with a positive charge. And as we all know, in this classic case of 'opposites attract', the hair will actually stick to the balloon. This attraction between oppositely charged objects is what we know as 'static'.
(republished on 07/13/06)