In liquids and gases,the inter molecular space is large and the inter molecular force of attraction is less.because of this the atoms of liquids and gases can move.they are so called fluids.
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Gases and liquids are considered fluids because they both have the ability to flow and move. The molecules comprising both gases and fluids are constantly in motion in random formations. The molecules continuously collide with each other and the sides of any container in which they are placed. They don't have definite volume and they are compressible because they have weak intermolecular forces of attraction.