Diffusion happens in all states of matter, from solid to liquid to gas. Diffusion occurs the quickest when matter is in its gaseous state. Diffusion is, quite simply, the movement of molecules from a very busy, or "concentrated," area to one of less concentration. Matter seeks to spread out instead of being crowded together, much as humans do. When matter is in a solid state, there isn't much room to spread out. The molecules are packed tightly together. In a liquid state, molecules have a bit more space. They can move around and find what we would consider a comfortable space with room to breathe. In a gaseous state, matter has the most freedom. Conditions, such as temperature, are perfect for molecules to quickly and easily spread out and about.