Here's why:

* Diffusion is the general movement of molecules from an area of high concentration to an area of low concentration. This movement happens because molecules are in constant random motion and tend to spread out.

* Osmosis is a specific type of diffusion where the movement of molecules is across a semipermeable membrane. This membrane allows certain molecules to pass through while blocking others. In the case of osmosis, it's the movement of water molecules across a membrane from an area of high water concentration to an area of low water concentration.

So, osmosis is a special case of diffusion where the movement of molecules is specifically the movement of water across a semipermeable membrane.