* Cells: The basic, fundamental units of life. They are the building blocks of all living organisms. Each cell performs specific functions, such as providing energy, storing information, or transporting materials.

* Tissues: Groups of similar cells that work together to perform a specific function. Think of tissues as "teams" of cells. Examples include:

* Epithelial tissue: Covers surfaces, lines cavities, and forms glands (skin, lining of the digestive tract)

* Connective tissue: Supports and binds other tissues (bone, cartilage, blood)

* Muscle tissue: Contracts to produce movement (skeletal, smooth, cardiac)

* Nervous tissue: Transmits signals throughout the body (brain, spinal cord, nerves)

* Organs: Structures made up of different tissues that work together to perform a complex function. Organs are essentially "teams" of tissues. Examples include:

* Heart: Composed of muscle tissue (for pumping), connective tissue (for structure), epithelial tissue (lining the chambers), and nervous tissue (to control contractions). Its function is to pump blood throughout the body.

* Lungs: Composed of epithelial tissue (for gas exchange), connective tissue (for support), muscle tissue (for breathing), and nervous tissue (to control breathing). Its function is to take in oxygen and expel carbon dioxide.

In summary:

1. Cells are the simplest units of life.

2. Tissues are groups of similar cells working together.

3. Organs are groups of different tissues working together to perform a complex function.

This hierarchical organization is a fundamental principle of biology and is essential for the proper functioning of all living organisms.