Ever since people began to observe the world carefully, they have classified matter into three main states - solids, liquids (pp. 18-19), and gases (pp. 20-21). A piece of solid matter has a fairly definite shape, unlike a liquid or a gas. Changing the shape of a solid always requires a certain amount of force, which can be either large or small. Squeezing or stretching a solid can change its volume (The amount of space it takes up) but generally not by very much. when they are heated, most solids will turn to liquid, then to gas as they reach higher temperatures. However, some solids such as limestone (pp. 36-37) decompose when they are heated. Crystals (pp. 14-15) and metals (pp. 16-17) are two of the most important kinds of solid.