Here's why:

* Osmosis is the movement of water molecules across a semi-permeable membrane from an area of high water concentration to an area of low water concentration.

* Semi-permeable membranes allow some substances (like water) to pass through but block others (like large molecules).

* Cell size changes occur because water moves to balance the concentration of solutes (dissolved substances) inside and outside the cell.

In summary: The water molecules moving across the membrane in your example are trying to equalize the concentration of water on both sides, which causes the cell to swell or shrink depending on the direction of water movement.