Here's why:

* Organs are complex structures that perform specific functions in the body.

* To carry out these functions, organs require the coordinated work of different tissue types. These tissues work together to create a functional unit.

* For example, the heart: It has muscle tissue for pumping, connective tissue for structure, epithelial tissue for lining chambers, and nervous tissue for regulating heart rate.

So, while a tissue is a group of similar cells working together, an organ is a group of different tissues working together to perform a specific function.