Here's why:

* Kinetic Molecular Theory: This theory states that all matter is made up of tiny particles in constant motion. The motion can be translational (moving from one place to another), rotational (spinning), or vibrational (atoms within the molecule vibrating).

* Temperature and Motion: Temperature is a measure of the average kinetic energy of the molecules in a substance. As temperature increases, the molecules move faster.

* States of Matter: The type of motion varies depending on the state of matter:

* Solids: Molecules vibrate in fixed positions.

* Liquids: Molecules can move around and rotate, but they are still close together.

* Gases: Molecules move freely and independently, with high translational motion.

* Absolute Zero: At absolute zero, all molecular motion theoretically stops. However, reaching absolute zero is practically impossible.

In summary: While the speed and type of motion vary, molecules are always moving unless they reach the theoretical state of absolute zero.