Here's why:

* Osmosis is the movement of water molecules across a selectively permeable membrane from a region of high water concentration to a region of low water concentration.

* Selectively permeable membranes allow some substances to pass through while blocking others. Cell membranes are a prime example.

* Change in cell size is a direct consequence of osmosis. If water moves into the cell, it will swell. If water moves out of the cell, it will shrink.

Let me know if you'd like to explore other aspects of osmosis, like the concepts of tonicity or how different environments affect cell size!