Silk is produced from jelly substances secreted by silkworms. The secretion hardens as soon upon exposure to air, forming what is known as a filament. The hardened filament is tightly held together by sericin, a silk gum. The silkworm spins layers of the single filament and a cocoon is formed. Each cocoon is composed of a single thread, measuring approximately 914 meters (3,000 feet). A series of hot and cold water immersions soften the silk gum and loosen the filament, leaving it ready to be reeled and spun into a silk thread. In instances where the silk is used as a filling, softened filaments are manually stretched and layered to form the comforter batting.