Liquid and gas are the phases of matter that take the shape of container.
This is very simple to imagine, if we have a piece of rock and we put it in a container, it will not take the shape of container as it already has a definite shape and volume. Liquid when put in a container takes the shape of container but varies in volume as per the container.
However, the gas phases is the phase of matter that perfectly takes the shape of container and occupies all the volume of container as well.
If we recall Dalton's Law of Partial Pressures we can see that the pressure exerted by the gas components in a container is same like the pressure exerted by the gas alone. These partial pressures of the component of gas combine in such a way that they exert total pressure equal to the constituents' pressure on the container. This way gases occupy all the volume of a container and take the shape of a container they're placed in.
