Here's why:

* All living organisms are made up of cells. Whether it's a single-celled bacteria or a complex human being, all life forms are composed of cells.

* Cells carry out the fundamental processes of life. They are responsible for metabolism, reproduction, growth, and responding to their environment.

* Cells are self-contained and self-sustaining. They have their own structures and systems that allow them to function independently.

Therefore, the cell is considered the fundamental building block of life.