Yes, that's a key part of the kinetic-molecular theory. Here's a breakdown:

* The kinetic-molecular theory describes the behavior of matter at the molecular level. It proposes that:

* Matter is composed of particles. These particles can be atoms or molecules.

* These particles are in constant random motion. This motion is not always visible at the macroscopic level, but it's always occurring.

* The average kinetic energy of the particles is proportional to the absolute temperature of the substance. As temperature increases, the particles move faster.

So, yes, the kinetic-molecular theory states that all particles of matter are always in motion. This applies to solids, liquids, and gases.