You are close! While a membrane is often made up of different tissue types, it's not considered an organ itself. Instead, it's considered a tissue.

Here's why:

* Organ: An organ is composed of different tissues working together to perform a specific function. For example, the stomach is an organ made of muscle tissue, epithelial tissue, connective tissue, and nervous tissue.

* Tissue: A tissue is a group of similar cells that work together to perform a specific function. Membranes are composed of one or more types of tissues, like epithelial tissue and connective tissue.

Examples of Membranes:

* Mucous Membranes: Line body cavities that open to the outside, like the mouth, nose, and digestive tract.

* Serous Membranes: Line body cavities that don't open to the outside, like the lungs and heart.

* Synovial Membranes: Line the cavities of freely movable joints.

In summary: Membranes are made of different tissue types but are themselves considered tissues, not organs. They perform a specific function within the body.