It eventually evolved into the Yangshao culture (5000 to 3000 BC), and their stone tools were polished and highly specialized. They may also have practiced an early form of silkworm cultivation. The main food of the Yangshao people was millet, with some sites using foxtail millet and others broom-corn millet, though some evidence of rice has been found. The exact nature of Yangshao agriculture, small-scale slash-and-burn cultivation versus intensive agriculture in permanent fields, is currently a matter of debate. Once the soil was exhausted, residents picked up their belongings, moved to new lands, and constructed new villages. However, Middle Yangshao settlements such as Jiangzhi contain raised-floor buildings that may have been used for the storage of surplus grains. Grinding stones for making flour were also found.