Question

why do gases hv neither a fixed shape nor a fixed volume

Answer 1

This is because of the fact that the intermolecular forces between the gases are very weak. Therefore the molecules of the gases are moving randomly and freely in all the directions.The container in which gases are kept can fill the whole volume of the container irrespective of the shape of the container because the molecules randomly dispersed in all the directions. That is why gases do not have fixed volume.