Dyeing is the process of applying colors to fabrics. A human practice for over three thousand years, it enabled the natural white color of silk, wool and cotton to be enhanced with a varied spectrum of color. Natural dyes are created from a myriad of sources, including shells, animals and plants, though these were not always permanent and frequently expensive. Modern dyes are based around artificial or processed substances such as coal and other chemicals. One benefit for a nation with an extensive dye industry is that the processes involved in creating dye can be easily retooled to other purposes. Germany discovered this in World War I when it was determined that factories producing artificial dye could be easily modified to create explosives and other tools of war.