The property that proves molecules are always moving is diffusion.

Here's why:

* Diffusion is the movement of molecules from an area of high concentration to an area of low concentration.

* This movement happens because molecules are constantly in motion, bumping into each other and spreading out.

* We see diffusion in everyday life: a drop of food coloring spreading in water, the scent of perfume filling a room, or the smell of freshly baked bread.

While other properties like temperature and pressure are related to molecular movement, diffusion directly demonstrates the constant motion of molecules.