Here's why:

* Function: Contractile vacuoles are found in some single-celled organisms, like amoebas and paramecia. They act like tiny pumps, collecting excess water from the cytoplasm and then expelling it out of the cell. This is crucial for maintaining the cell's water balance and preventing it from bursting.

* Mechanism: They work by accumulating water from the cytoplasm and then contracting, squeezing the water out through a pore in the cell membrane.

Important Note: Not all cells have contractile vacuoles. They are primarily found in freshwater organisms, where the concentration of water outside the cell is higher than inside. This creates a tendency for water to move into the cell through osmosis, which the contractile vacuoles counteract.