We put some styrofoam beads into (a) a plastic container; and (b) a metal container. We then put these containers on top of a Van de Graaff generator and charge it up. Look what happens:
In the second case, where the beads are in the conducting container, nothing happens because of the shielding. In the upper video, where the beads are in the insulator, the beads become negatively charged and are repelled from the also negatively charged surface of the Van de Graaff generator.