Why do gases fill the container they are in , rather than staying in just one part of it

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.  

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.

The law of conservation of mass is like a burning candle

Explanation:

1 example picture a regular candle with wax