A gas cylinder cannot be half filled because of the following reasons:
1. Gases are the fluid form of matter which occupy the entire container.
2. Gases have high kinetic energy and
Very very less inter molecular force.
3. Gases have a property that they can diffuse to all the corners of container and occupy entire space of the container.
4.if we try to half fill , it occupies full space.
5. For example: when we blow air into balloon, it tries to occupy full space of balloon.