Here's why:

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

* Organs: Structures made of different tissues that work together to perform a more complex function. Examples include the heart (made of muscle, nervous, and connective tissue), the stomach (made of muscle, epithelial, and connective tissue), and the brain (made of nervous tissue).

So, while tissues are the building blocks of organs, organs are the functional units that perform complex tasks in the body.