The motion of two substances by the random movement of their molecules is called diffusion.

Here's why:

* Diffusion is the process where molecules move from an area of high concentration to an area of low concentration. This movement is driven by the random motion of molecules, which constantly collide with each other and spread out.

* This movement happens in both liquids and gases, and even to a lesser extent in solids.

* When two substances are in contact, their molecules will intermingle due to this random motion.

Example: If you drop a drop of food coloring into a glass of water, the food coloring will slowly spread out throughout the water due to diffusion.