Here's why:

* Cell Division: Cells replicate themselves through a process called mitosis. This creates new cells, increasing the overall size and number of cells in the organism.

* Cell Differentiation: As cells divide, they specialize into different types of cells with specific functions. This process is called differentiation. For example, some cells become muscle cells, while others become nerve cells or skin cells.

These two processes working together allow organisms to grow larger and more complex, developing different tissues and organs.