Mold play an important part in nature by breaking down dead organic matter such as fallen leaves and dead trees; indoors, mold growth should be avoided. Mold reproduce by means of tiny spores. The spores are like seeds, but invisible to the naked eye, that float through the air. Mold may begin growing indoors when spores land on moist surfaces. There are many types of mold, but all require moisture and a food source for growth.