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. For example, muscle tissue contracts, nervous tissue transmits signals, and epithelial tissue forms linings.

* Organ: A structure made up of different tissues that work together to perform a more complex function. For example, the heart is an organ made of muscle tissue, nervous tissue, and connective tissue.

* Organ System: A group of organs that work together to perform a major bodily function. For example, the digestive system is made up of the stomach, intestines, and other organs.

So, the process goes from individual cells to a tissue, then to an organ, and finally to an organ system.