Here's why:

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

* Tissues: Groups of similar cells that work together to perform a specific function. For example, muscle tissue, nervous tissue, epithelial tissue, and connective tissue.

* Organs: Structures made up of different tissues that work together to perform a complex function. For example, the heart, lungs, stomach, and brain.

* Organ Systems: Groups of organs that work together to carry out major bodily functions. For example, the digestive system, circulatory system, and nervous system.

So, tissues are the intermediate level of organization between cells and organs, and they are the building blocks of organs.