There are four fundamental forms of matter: solids, liquids, gases, and plasma. Solids have a definite shape and volume due to their rigid molecular structure. Liquids also have a definite volume but cannot maintain a definite shape as their molecules are drawn together weakly, allowing liquids to flow and take the shape of their container. Gases have neither a definite shape nor volume as their molecules are very weakly bonded and spread out to fill their container. Plasma differs from solids, liquids, and gases in that its molecules are ionized, meaning they are positively or negatively charged, and thus exhibit electromagnetic properties.