Here's why:

* Clarity: Water is transparent, allowing light to pass through it easily, ensuring a clear view of the cells.

* Preservation: Water helps to maintain the shape and structure of the onion cells, preventing them from drying out and shrinking.

* Accessibility: Water is readily available and inexpensive, making it a practical choice for classroom experiments.

While water is the most common choice, you can also use other solutions like iodine solution to stain the cells and make certain structures more visible.