Code systems that will keep your deepest secrets very secret

NAC YUO IFGRUE HTSI OTU?

Sure you can -- just unscramble the letters.

?SIHT TUOBA WOH -- try reading it backwards.

Codes like these are fun and easy to figure out, and that's just the problem. But here are some more challenging code systems you can try:

* Shift code. One that is easy to do -- but hard for others to decode -- dates back to Roman days. It's called a ''shift code'' because the letters shift along the alphabet, and it goes like this:

Across the top of a sheet of paper, write out the alphabet:

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

Now, put your finger on the ''A,'' and count four more letters to ''E.'' Start a new alphabet over the E:

A B C D E F GH I J K L M N O P Q R S T U V T W X Y Z

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

You will have four letters left over. Put them over your first four letters in your real alphabet, so your code looks like this:

W X Y Z A B C D E F G H I J K L M N O P Q R S T U V

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

To ''encode'' (or put a message into code), look at the bottom alphabet to find your real letters, and write them in their code letters found in the top alphabet. That way, A would be W, B would be X, and YES would be UAO. Try putting this into code:

MEET ME AFTER SCHOOL.

Then try to decode this one by looking at the top alphabet for the code letter and seeing what the real letter is in the bottom alphabet:

UKQ WNA OK OIWNP!

The shift code given here is a shift of four, but you can choose any number. Just be sure to let the other person know what the number is!

* Dot code. Codes don't always use letters. Here is one that uses dots:

Take a piece of lined paper and turn it sideways. Write an alphabet in between the lines across the side of the paper and cut it out. This is your decoder.

Now lay the decoder across the side of another sheet of lined paper so that the lines match up. Then put a dot under the first letter you want to put in code. Move your decoder down a little so it covers your first dot. Then put another dot under the second letter you want to encode, and so forth. This is what ''Let's play soccer'' looks like in dots:

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

[TEXT OMITTED FROM SOURCE]

You and your partner can use a scrambled alphabet, or a shift alphabet, for double secrecy with this code.

* Wheel code. Another code uses a cardboard tube for a decoder. You can get the tube from inside a roll of toilet paper or paper towels. You will also need some long strips of plain paper.

Wind a strip of paper around the tube and hold it there with your left hand. With your right hand, write a message across the paper on the tube. Long messages work best. Then take the paper off the tube. Your message will be all mixed up, but your partner can decode it by winding it around his cardboard tube.

* Fence post code. Here is a complicated code. You should know a little bit about division before trying it. It is called the ''fence post code'' because the letters are written like they are sitting on fence posts.

Here is how to put ''There is a frog in my desk'' into this code. You put the first letter (T) on the first row on your paper, the second letter (H) on the second row, the third letter (E) back on the first row, the fourth letter (R) down on the second row, and up and down until it looks like this:

T E E S F O I M D S

H R I A R G N Y E K

Now you can write your code by combining the first row (TEESFOIMDS) with the second (HRIARGNYEK) to read: TEESFOIMDSHRIARGNYEK.

If your friend wants to decode this message, he must first count the total number of letters in the code. There are 20 letters in this one. Then he should divide that number by two.

Since the answer is 10, he writes the first 10 letters across the first row on a piece of paper with spaces between the letters:

T E E S F O I M D S

Then he puts the second 10 letters on the second row underneath the spaces of the first row:

T E E S F O I M D S

H R I A R G N Y E K

Now he must figure out the message by reading up and down the lines. See what you can do with this message: YUADCDTIOCNEOEHS.

These examples use an even number of letters. If your message has an odd number, add an extra throwaway letter like X or Z to make the division work out evenly.

There are many more kinds of codes. If you want to learn more, look in the card catalog in your library under ''Codes'' or ''Ciphers.''

UYO ANC OD IT!