Here's why:

* Insulators resist the flow of electricity. This is because their electrons are tightly bound to their atoms and don't move easily.

* Conductors allow electricity to flow through them easily. This is because their electrons are loosely bound and can move freely.

Styrofoam is made of expanded polystyrene, a polymer with a very low density. The structure of its molecules doesn't allow for easy electron movement, making it an excellent insulator.