Paper is thin material habitually used for writing upon, printing upon or packaging. It is formed by the amalgamation of fibers, characteristically vegetable fibers composed of cellulose, which are subsequently held together by hydrogen bonding. While the fibers are generally natural in origin, a wide variety of synthetic fibers, such as polypropylene and polyethylene, may be incorporated into paper as a way of imparting desirable physical properties. The most general source of these kinds of fibers is wood pulp from pulpwood trees. Vegetable fiber resources such as cotton, hemp, linen, and rice are also used.