Show a one dollar bill to your audience…
Question: What is it?
Possible Answers: A Dollar, Paper Money, etc.
Question: What is it worth?
Possible Answers: One Dollar!
Question: But it is just a piece of paper with some printing on it! Why is it worth a Dollar?
…Your audience will (hopefully) get stumped on this one, because there is really no reason why a dollar is worth a dollar other than because everyone agrees that it is worth a dollar.