Considering this prompt, you may want to think about the qualities, ideas, and values that characterize life in the 21st century.
Many people believe that life is becoming faster, more complex, and more socially connected. This increase in social connection is facilitated by growing urban populations (cities) and the Internet (technology), yet due to the practical means of communication as they now exist (computers and cell phones) some people say that the "social connections" of today are merely virtual.
If any of these ideas strike you as agreeing with your own outlook, you might begin to see which concepts to draw on when you construct your metaphor. If you believe the world is moving faster and faster, maybe a fast animal or a bullet train would symbolize this idea. If you believe that the world is growing more connected, you might use a spider web, a computer network diagram, or a computer chip to represent this idea.
If these ideas do not resonate with you, you may want to try the thought experiment yourself, asking questions like "How to do I see the world?", "How is the world changing or different now from how it once was?", etc.