Here's why:

* Cells are the basic unit of life: They are the smallest functional unit that can carry out all the processes necessary for life, such as metabolism, growth, and reproduction.

* Tissues are made of similar cells: A tissue is a group of similar cells that work together to perform a specific function. For example, muscle tissue is made of muscle cells that contract to allow movement.

* Organs are made of different tissues: An organ is a structure made of two or more different tissues that work together to perform a complex function. For example, the stomach is made of muscle tissue, epithelial tissue (lining), and connective tissue (supporting structure), all working together to digest food.

So, while cells are the smallest unit, they are the fundamental building blocks that make up tissues, which in turn make up organs.