The gases which is filled in the container does not stay in a part of container because of the fact of the properties of the gaseous matter.
Step-by-step explanation:
The gaseous matter have the property to have no fixed shape or volume. This is in turn because their atoms or molecules are not compact in a form, they have a large inter molecular space which makes the atoms to be spread.
The inter molecular space is large because the inter molecular attraction forces are weak. Therefore they are free to move or vibrate at a high speed and thus when poured into a container, occupied the whole space.