The three states of matter are solid, liquid and gas. Each state of matter is defined by physical characteristics such as its shape, volume and compressibility. Changing from one state of matter into another is accomplished by altering the temperature, pressure or composition of a substance.
Solids are rigid substances that have definite shapes and volumes. The molecules in a solid are rigidly positioned close to one another, so the molecules cannot move around much. Liquids have defined volumes but no actual shape. Liquids take the shape of the container they inhabit, and their molecules flow past one another but are still touching each other. Gases have neither defined shapes nor volumes. Molecules in gases do not lie against one another; instead, wide spaces separate the gas molecules and they move past each other freely.