Andrea Wolk, age 9, of Sarasota, Florida, for her question:

Why are coin edges milled with ridges?

Suppose you pay for a nickel candy with a quarter. If you get back only 15 cents, you have been shortchanged. This may be a mistake. But you just may be shortchanged on purpose, Tricks of this sort are nothing new. Sneaky Pete and other crooks have been trying to snitch more than their rightful share since coins were invented. In olden days coins were made of gold and silver, and each one was a certain weight. Wouldn't you know, the Sneaky Petes scraped bits of precious metal from the edges. They stole dishonest fragments from those honest coins. The coins passed around from hand to hand got smaller and lighter. Bit by bit, they lost their worth and value.

But honest people always were smarter than crooks. They figured a way to stop Sneaky Pete's trick. They milled groovy ridges around their honest coins. That way they could see at once whether the edges had been shaved. What's more, it made the coins less slippery and easier to handle. We still mill the edges of some of our coins, though they are no longer made entirely from valuable metals. But Sneaky Pete and other crooks are still around to shortchange people with other dishonest dodges.