Here's a breakdown:

* Cells: The basic building blocks of life.

* Tissues: Groups of similar cells that work together to perform a specific function. For example, muscle tissue contracts to allow movement, nervous tissue transmits signals, and epithelial tissue forms linings and coverings.

* Organs: Structures made up of different tissues that work together to perform a more complex function. For example, the stomach is an organ made up of muscle tissue, epithelial tissue, connective tissue, and nervous tissue.

* Organ Systems: Groups of organs that work together to perform a major body function. For example, the digestive system breaks down food, the circulatory system transports blood, and the nervous system controls and coordinates bodily functions.

So, cells working together are the first step in building the complex structures and functions of living organisms!