Question

Why should we put water in order to make observations with a microscope?

Answer 1

Water or oil are often used to provide a buffer between a high-magnification lense and the sample.

Explanation:

A thin layer of liquid may also help to stabilize the sample on the microscopy plate for easier adjustment for viewing.

At high magnifications it is often necessary to place the magnifying lense very close to the subject. Too close can distort - or even damage - the sample. Too far makes focusing difficult.

Placing a thin film of water or oil over the sample allows the lense to closely approach the sample without touching it or changing the refractive index of the medium.

https://www.microscopyu.com/microscopy-basics/water-immersion-objectives

http://www.microscope-microscope.org/basic/preparing-microscope-slides.htm