Here's why:

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

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

For example:

* The heart: An organ responsible for pumping blood throughout the body.

* Cardiac muscle tissue: One of the tissues that makes up the heart, specifically responsible for the heart's contractions.

Therefore, the functional unit of an organ is the tissue because it's the smallest unit within an organ that can perform a specific function.