Composite materials, or simply composites, consist of two parts; the reinforcing phase and the binder or matrix. Composites may be natural substances, such as wood and bone, or man-made substances. A composite product is different from each of its components and is often superior to each individual component. The binder or matrix of a composite is the material that supports the reinforcing phase. The reinforcing phase is usually in the form of particles, fibers, or flat sheets. Reinforced concrete is an example of a composite material. The steel rods embedded in the concrete (the matrix) are the reinforcing phase adding strength and flexibility to the concrete. High-performance composites are composites that perform better than traditional structural materials, such as steel. Most high-performance composites have fibers in the reinforcing phase and a polymer matrix. The fibers may be glass, boron, silicon carbide, aluminum oxide, or a type of polymer. The fibers are often interwoven to form bundles. The purpose of the matrix, usually a polymer, in a high-performance composite, is to hold the fibers together and protect them.