Here's why:

* Cells are the basic unit of life: They are the smallest structures capable of carrying out all the processes necessary for life, such as metabolism, growth, and reproduction.

* Unicellular organisms: Some organisms, like bacteria and amoebas, are made up of just a single cell.

* Multicellular organisms: More complex organisms, like plants and animals, are made up of trillions of cells organized into tissues, organs, and organ systems.

Therefore, everything that is considered "alive" is made up of cells.