Under the KMT assumptions, gases consist of a collection of tiny hard spheres.

The mental images conjured up by hard spheres are not those we see in gases.

It is very hard to compress a liquid or solid very much. Gases are readily compressed.

The volume of most liquids and solids increases a little bit when the temperature is increased. When the temperature increases, gases usually expand quite a bit.

Three properties are used to describe samples of gases: volume, temperature, and pressure [local] (original page by James Plambeck no longer available).