Polymers are long molecular chains made of repeating monomers. They can be thermosets that permanently harden, or thermoplastics that soften when heated. Composites contain fibers embedded in a polymer matrix to achieve properties neither material has alone. Fiber reinforced plastics are composites with fibers like glass, carbon, or aramid in a plastic matrix. The fibers increase strength and stiffness while the matrix binds them and transfers stress. Composites find applications where high strength and low weight are required.