Most living things are made up of cells. This is a fundamental principle of biology known as the cell theory.

Here's why:

* Cells are the basic unit of life: They carry out all the essential functions necessary for life, such as metabolism, reproduction, and responding to stimuli.

* Organisms are built from cells: Simple organisms, like bacteria, are single-celled. More complex organisms, like humans, are made up of trillions of cells organized into tissues, organs, and organ systems.

Exceptions:

While the vast majority of living things are cellular, there are a few exceptions, such as viruses. Viruses are not considered living organisms because they lack the ability to reproduce on their own and require a host cell to do so.