Here's why:

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

* Organs are made up of different types of tissues that work together to perform a more complex function. The heart is made up of muscle tissue (to pump blood), connective tissue (to provide structure), nervous tissue (to regulate heartbeat), and epithelial tissue (to line the chambers).

So, while the heart is composed of different tissues, it functions as a whole unit to circulate blood throughout the body, making it an organ.