Here's why:

* Cells are the basic unit of life: They are the smallest structures that can carry out all the processes necessary for life.

* Organisms are made of cells: Whether it's a single-celled bacteria or a complex human being, all living things are made up of one or more cells.

* Cells perform all life functions: From taking in nutrients to producing energy, cells are responsible for all the activities that keep organisms alive.

So, if something is alive, it's made of cells!