You guys don't know what money is or about buying things? [Oh. Well. She had never had to explain this sort of thing before, but she had to give it a shot. It took a few long moments of thinking, though.]

Let's say that I have a red ball and you guys have a yellow ball. If I wanted a yellow ball and you wanted a red one, then we could just trade, right? But if I didn't want a yellow ball and I only wanted to trade for a green one, no one gets anything. That's where money comes in! You could give me money, and then I would give you my red ball. After that, I could use that money to get a green ball from someone else.

And when you give someone money and get the thing you want in return, that's called buying something! Does that make sense?