Do you or someone you know live in a large city? A city is a community of people who work together to support each other and help make life possible. Each person has a different job. Some are builders, others work in markets, while others clean up. Every job is important. Suppose you work in an office building on the 86th floor and decide that you are hungry. How will you get food?
You walk down the narrow hallway, climb into an elevator, and go down to the main floor. From there you walk out into the street and continue towards your favorite lunch spot. Using hallways, elevators and roads, you eventually find your way to your lunch.
In many ways, your body is like a city. Instead of millions of people, your body is made up of trillions of cells, each with a different job. What happens if a cell on the 86th floor of your body gets hungry? How will it get food?