Hide and Seek!

Here's an example of a code that hides within another message. It's called a Cardan grille. Read the message below. Does it seem strange? No, it's just a note from one person to another, making plans to go to the zoo.

Aahh, but there is a secret message hidden within all of those words! It can be read with a stencil, or grille, with cutouts that reveal only some of the letters. The grille is placed over the original message, and the hidden words suddenly appear. Here is the grille for this message (the black areas are the cutouts):

OK, so maybe that's not very helpful by itself. But if we place the grille over the original message, the mystery is solved:

How about that?

The Cardan grille is a very old type of code, invented by Girolamo Cardano in 1550. It is an example of steganography, which means "concealed writing". Unlike many other codes, no one would even suspect that the original message held a secret meaning. The problem with the Cardan grille is that both the sender and the receiver must have identical copies of the grille. Also, it can be hard to construct a natural-sounding message around the necessary letters.

Try making some Cardan grilles yourself. You might want to start with hiding just a few words, rather than a whole sentence.