is a special type of diffusion where only water moves across a partially permeable membrane

The process you're referring to is called Osmosis.

Here are the steps to explain it:

1. Osmosis is a special type of diffusion, which is a process that involves the movement of particles from an area of high concentration to an area of low concentration.

2. In the case of osmosis, the particles that are moving are water molecules.

3. The movement of water molecules occurs across a partially permeable membrane. A partially permeable membrane is a barrier that allows certain substances to pass through it, but not others. In this case, it allows water molecules to pass through, but not other types of molecules.

4. The water molecules move from an area of high water concentration (where there is a lot of water) to an area of low water concentration (where there is less water).

5. This movement of water molecules continues until there is an equal concentration of water on both sides of the membrane. This is when osmosis stops.

6. Osmosis is a vital process in biological systems, as it helps to regulate the water content in cells and can also be involved in the absorption of water in the digestive system.