In physics, gases are considered to be liquids, along with plasma and plastic solids. This is because these states can be molded and transformed as needed. Gases and liquids flow and move freely and randomly. The more heat that is applied to either, the faster their molecules move which will cause them to expand.
Although there are fundamental differences between the two, they are both considered fluids for physics' purposes.
In physics, a fluid is a material that continually deforms under applied shear stress. Fluids are a subset of the phases of matter. These include liquids, gases, plasmas, and plastic solids. Gases and liquids are both seen as fluids because they flow and move.
The molecules in both gases and fluids are constantly moving randomly. The molecules continually collide with each other and the sides of any container in which they are placed. Liquids always take the shape of the receptacle in which they are, but they do not increase in volume to fill the container.