A group of cells that are organized into a layer or mass constitutes a tissue.

Here's why:

* Cells: The basic building blocks of all living organisms.

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

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

* Organ System: A group of organs that work together to carry out major bodily functions.

So, when cells come together in a structured way to perform a specific task, they form a tissue.