How do you explain a bicycle to a child?

Imagine you're trying to introduce the concept of cryptocurrency to a young child who's never heard of it before. How would you go about explaining it in a simple and easy-to-understand way, using analogies or real-life examples that they can relate to? Remember, you want to keep it fun and engaging, while still conveying the basic idea of what cryptocurrency is and how it works. So, how would you explain cryptocurrency to a child?