Here's why:

* Cells: The basic building blocks of all living things.

* Tissue: A group of similar cells that work together to perform a specific function.

* Organ: A structure made up of different tissues that work together to perform a more complex function.

* Organ System: A group of organs that work together to carry out a major bodily function.

* Organism: A complete living being made up of organ systems.

So, the process goes from individual cells to tissues, then to organs, organ systems, and finally, a complete organism.