Find an example in the real world that you can separate into objects that work together to achieve its goal. How would you describe each of the objects and what does each of them do? This might be software or it could be any physical object that has interacting components.
A car. There are too many objects to list but in its simplest form:
The list goes on. They all need to interact with each other in order for the car to move. A very simple analogy that I would use with a younger class perhaps.