What it shows:
Bringing a charged rod close to neutral dielectric polarizes the dielectric's surface charges. Here a pile of Styrofoam puffs are polarized and attracted to a charged rod.
How it works:
The neutral puff experiences a non-uniform electric field from the rod. Although there are polarized charges of both kinds, because (figure 1) the field is stronger near the rod due to the concentration of positive charges, there is a net attraction. On a dry day they'll jump to meet the rod.
figure 1. Inducing a dipole
Setting it up:
Use the Teflon/fur or the glass/silk from the Triboelectric Effects demo, and a pile of Styrofoam puffs (a rich source being environmentally unfriendly packaging material). On a humid day the fur should be dried under a heat lamp.
The Teflon and rabbit's fur combination works best, but it is good to use both to show that the polarity of the inducing rod doesn't matter.