James: When spies write messages, they do it so only other spies can read it.
Henri: How?
James: Sometimes, they use invisible ink.
Henri: Oh, I love invisible ink! It shows up when you heat it.
James: Right. Others use a piece of paper with holes in it. See? If you read only the words you can see in the holes, you get the secret message. Sometimes, they write their message in coded numbers that refer to a specific page, sentence, and word in a book. That person that gets the coded numbers has a copy of the same book.
Henri: Oh. So they use the numbers to find the words, and voilà! It spells out the secret message.
James: Mm-hm. Then there is the maskling. Anyone reading the regular letter doesn't know there is a secret message hidden in the writing.
Henri: How does it work?
James: Simple. The spy knows he's only supposed to read what he sees in the cutout. In this case, an hourglass.
Henri: Magnifique! But I will invent the unbreakable Henri code. Paper. Paper. I need paper!