Because In gases the molecules are losely packed which made them not to have a definite shape. This is the reason that we can fill more and more gas in a cylinders although it is filled with some gas before.
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If in a 2 litre cylinder, gas is filled and then that gas is kept in a 4 litre cylinder then it will not occupy a half part but it will occupy the full space present there.By this we can show that gas does not have definite volume and the reason behind this is that they have less intermolecular space between their molecules.
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