Here's why:

* Insulators have very few free electrons, making them poor conductors of electricity. Think of rubber or glass.

* Conductors have many free electrons, allowing electricity to flow easily. Examples are copper and silver.

* Semiconductors fall in between. They have more free electrons than insulators, but fewer than conductors. Their conductivity can be manipulated by adding impurities or applying an electric field. This makes them incredibly useful in electronics, as seen in transistors, diodes, and integrated circuits.

Common examples of semiconductors include silicon and germanium.