Here's why:

* Cells: The basic unit of life.

* Tissues: Groups of similar cells working together to perform a specific function (e.g., muscle tissue, nervous tissue).

* Organs: Made up of different tissues that work together for a common purpose (e.g., heart, lungs, brain).

* Organ Systems: Groups of organs working together (e.g., circulatory system, digestive system).

* Organism: A complete living being made up of multiple organ systems that work together to sustain life.

So, an organism is the ultimate and most complex level of organization in a living system.