"Hmmm." The money-changer squints at your coins and jingles them in his hand. "Shiny new coins...But no silver in them. One silver denarius is all I'll give you."

Good thing you had some change handy. In the ancient world, coins made of precious metals were widely used as currency. When the Roman government needed money, more coins would be minted with less silver. But making more money caused inflation, so the new coins lost value.

One important kind of money is made from a precious metal that has value because it's rare.