➢ Rigid means ‘unbending’ or ‘inflexible’. A stone is rigid because it is unbending or inflexible. Fluid means ‘a material which can flow easily’ and requires a vessel (or container) to keep it. 

➢ A solid is a rigid form of matter. Due to their rigidity, solids have a tendency to maintain their shape when subjected to outside force. Thus, the main identifying characteristic of solids is their rigidity. Solids do not require a container to keep them. A brick and a log of wood are two common solids. 

➢ A liquid is a fluid form of matter which occupies the lower part of the container. Being fluids, liquids require a container to keep them. A liquid has a well-defined surface, so a liquid can be kept in an open container. The liquid will not escape from the open container by itself. Water and milk are two common liquids around us. 

➢ A gas is a fluid from of matter which fills the whole container in which it is kept. Being fluids, gases also require a container to keep them. A gas does not have a free surface, so a gas has to be kept in a closed container. A gas will escape if kept in an open container. This is why gases are usually kept in air-tight gas cylinders. For example, cooking gas (LPG) is kept in air-tight metal cylinders. From this discussion we conclude that both, liquids and gases are known as fluids. Characteristic of liquids and gases of ‘flowing’ easily is called ‘fluidity’. Due to fluidity, liquids and gases change their shapes readily when subjected to outside force.