Meet Paige. Press play to watch the video.
Your mission
Paige the bookworm has read every book in the library. She knows which ones hide secret messages. Today you will learn how spies turn a book into a code.
- Watch Paige's video.
- Open each tab, from left to right.
- Earn four badges to unlock The Locked Library.
Take your time. If you miss a question, you get a new one, so think before you click.
My badges
Teacher note
- Built for grades 3 to 5. Plan on 40 to 50 minutes, or two shorter sessions.
- The page has its own six-page book, The Clock Tower Cat. Every message is hidden in that book, so no supplies are needed.
- Students cannot click until they get it right. A missed question swaps in a new question, and a missed message swaps in a new message.
- Messages are picked at random, so neighbors will not have the same answers.
- The Locked Library is a team challenge. It works best with a partner and scratch paper.
- The Book Machine, a free encoder and decoder, stays locked until the library is open. That way nobody can use it to skip the practice.
- Offline extension: partners pick a page from a class novel or a dictionary and trade notes. Both partners must use the same copy, which makes a good talking point about editions.
- History on this page: Benedict Arnold used a book cipher in the American Revolution, and the Beale papers are a famous treasure code that is still mostly unsolved.
- Progress saves in the browser on this Chromebook only.
The key is a book
Most ciphers hide a message one letter at a time. A book cipher hides whole words.
Two friends each get a copy of the very same book. To send a word, you find that word in the book. Then you write down where it is. Your note is nothing but numbers.
Your friend opens their copy, follows the numbers, and finds the same word. The book is the key.
Three numbers for every word
Each word gets three numbers. The first number is the page. The second number is the line on that page. The third number is the word on that line.
3page2line5word means page 3, line 2, word 5.
Here is the book for this lesson. Click any word to see its three numbers.
Click a word in the book.
Same book, same pages
A book cipher only works when both copies match. The same words must sit on the same pages and the same lines.
A bigger copy of the story, or a newer one, can have different pages. Then the numbers land on the wrong words and the message turns into nonsense.
There is one more catch. You can only send words that are in the book. If your word is missing, you have to pick a different one.
Why spies loved it
A snoop who finds your note sees only numbers. To read it, they would need to know which book you used. There are millions of books in the world.
During the American Revolution, a general named Benedict Arnold secretly switched sides. He sent letters to the British with a book cipher, so nobody else could read them.
A smart spy has one more trick. A common word like THE shows up many times in a book. The spy can use a different THE each time, so the same numbers never repeat.
How to read a book code
- Look at the first group of numbers.
- Turn to that page in the book.
- Count down to the line. The red numbers on the side help you.
- Count across to the word, then click it.
How to write a book code
- Hunt through the book for the first word of your message.
- Type its page number, its line number, and its word number.
- Do the same for every word.
A word can be in the book more than once. Any spot with the right word works.
No book, no message
A code breaker cannot test every book in the world. But what if they find the note in a room with only three books? Then they can test each book until one of them makes sense.
There is a famous treasure note from the 1800s called the Beale papers. It has three pages of numbers. One page was solved with a famous American document as the key. Nobody has ever solved the other two.