When I was in high school, I had a friend who worked at a local hardware store. The things he learned from that job were amazing. For example, he knew exactly how much the store had paid for a certain type of air freshener called an "aroma disc player." The amount was printed, in secret code, on the price tag.

Using that code, I could figure out just how much money the hardware store made on people like me every time we purchased a new fragrance record, whether it was mint, cinnamon, or the ultra-pungent "man's world" variety.

The code was called CHARLESTON. There are 10 letters in this word, each one different. Each letter corresponded with a number from 1 through 10. Using logic (lots of prices end in .99, for example), my brothers and sisters and I were able to figure out which letter corresponded to which number.

We felt quite powerful knowing this secret about the store. And even though the place is no longer in business, and we no longer have the opportunity to buy new fragrance records (or fragrance CDs or MP3s, for that matter), I still find satisfaction in having cracked that secret code.

Secret codes--and the messages they communicate--are everywhere. Although they're probably most frequently thought of as military tools, you can find them on playgrounds, in computer software, on TV, and even in music.

Pig Latin? That's a rudimentary secret code. Or, should I say, "ecretsay odecay"? As with all secret codes, it disguises a message two people are communicating.

Because Pig Latin is broadly known and not very hard to figure out, it's not the greatest way of delivering secret information. Cryptography, on the other hand--which is Greek for secret writing--has been used for hundreds, if not thousands, of years to pass messages.

Although it's often used as a tool during wartime, the art of writing and decoding secret messages has lots of other applications. On the Internet, it can keep your personal data safe. At home, it can be a nice way of telling someone you love them--something you can do secretly, every day.

I've sent you a secret message in this introduction, by the way. To decode it, all you need to know are these numbers: 18, 55, 166, 175, 238, 371, 385.