The earliest humans had access to only a very limited number of ________ materials, such as : stone, wood, clay, skins, and so on . With time they discovered techniques for producing materials that had properties superior to those of the limited ones, and they discovered that the properties of a material could be altered by and by the addition of other , such as : pottery and various metals. It was not until relatively recent times that scientists came to understand the between the structural elements of materials and their properties, these include metals, plastics, glasses, and fibers.