Here's why:
* Isotonic: This means the concentration of solutes (dissolved substances) inside the red blood cell is the same as the concentration of solutes outside the cell in the surrounding solution. Since the concentration is equal, there's no driving force for water to move in or out of the cell.
* Hypotonic: This solution has a lower solute concentration than the inside of the red blood cell. Water will move into the cell to try to dilute the higher concentration inside, potentially causing the cell to swell and burst (lysis).
* Hypertonic: This solution has a higher solute concentration than the inside of the red blood cell. Water will move out of the cell to try to equalize the concentrations, causing the cell to shrink (crenation).