From one individual to an entire planet filled with human beings, each of us have a role to play. Some of us are airline pilots, teachers, doctors, farmers, builders, and more. Without a variety of people with specialized skills working together, our world would be very different. There would be no advancement of technology, no discovery, and medical and food technologies would not exist as they do today.

There is a similar organization within the bodies of living things. If we could go inside a living thing, the first thing we would see are what we call cells. Cells are like people in the example above. Each one is an individual entity working hard on specialized jobs. Instead of fire fighters, cells might be assigned the job of helping digest food, or carrying messages from the brain, or fighting diseases that enter the body. Cells work along side other cells in what we call tissues. This is similar to how individual people work inside a family, in the example above.