The components for synthetic fibers are produced from the natural resources petroleum and natural gas. These components are called monomers and the macromolecules made out of them are called polymers. The first synthetic fibers that could be manufactured were polyamides which due to their chemical structure are related to natural silk. Nylon and Perlon are thermoplastics. When heated they become soft and eventually melt. Polystyrene is also a thermoplastic. Another category of polymers are the duroplastics, where the monomers are not in chains but are spatially networked into polymers. These plastics are brittle and hard and do not change shape when heated.