Pith balls are little styrofoam (or similar material) balls coated in conductive paint. For example, pith balls might look like this:
When a charged rod is brought close to the pith balls, their charges polarize. If they come in contact with the charged rod, they can inherit the same sign charge. Since like charges repel. when similarly charged, the pith balls behave like siblings and refuse to touch each other.
And like the immature children that they are, they rebel against their parents, too.
Note: it is only charged pith balls that behave like this.
You can also use this idea to make larger and more fantastic demonstrations that fail to work correctly in a much larger and more fantastic way.