Here's why:

* Basic building block: Cells are the smallest units that can carry out all the processes essential for life.

* Self-contained: They have their own internal structures (organelles) that perform specific functions like energy production, waste removal, and protein synthesis.

* Diversity: While all living things are made of cells, there are many different types of cells with varying structures and functions. This diversity allows for the complexity and variety of life on Earth.

Think of it like this: Just like a brick is the fundamental unit of a building, a cell is the fundamental unit of a living organism.