A cell stays the same size in an isotonic solution because the concentration of solutes is equal inside and outside the cell. This means that there is no net movement of water across the cell membrane. Here's why:

* Osmosis: Water moves from areas of high water concentration (low solute concentration) to areas of low water concentration (high solute concentration) across a selectively permeable membrane.

* Equilibrium: In an isotonic solution, the water concentration inside the cell is the same as the water concentration outside the cell. This means there's no difference in water potential, so water moves equally in both directions across the membrane.

Think of it like this:

Imagine a cell as a balloon filled with water and solutes. If you put the balloon in a solution with the same concentration of solutes as the inside of the balloon, the water will move in and out at the same rate. The balloon won't shrink or swell because the water movement is balanced.

In contrast:

* Hypotonic solution: The solution outside the cell has a lower solute concentration than inside the cell. Water will move into the cell, causing it to swell and potentially burst.

* Hypertonic solution: The solution outside the cell has a higher solute concentration than inside the cell. Water will move out of the cell, causing it to shrink and shrivel.

Therefore, an isotonic solution creates a stable environment for cells by preventing them from gaining or losing too much water. This is essential for cell function and survival.