These are tiny tubes 10,000 times thinner than a human hair, but incredibly strong for their weight. The fact that they are hollow allows them to function as pipes for transporting atoms and molecules. Since they can be insulators, conductors, or semiconductors, they are used as molecular wires and circuits. They can also serve as tips for atomic force microscopes, or function as molecular bearings and springs in microbots or smart matter. The superior strength-to-weight ratio of carbon nanotubes permits light but extremely strong nanocomposite or nanofiber materials. These are used in products such as vehicle hulls, body armor, and space elevator cables.