Meet Cardano. Press play to watch the video.
Your mission
About 500 years ago, an Italian inventor named Girolamo Cardano hid secret messages with nothing but a card full of holes. Today you will use his trick, and then you will make a grille of your own.
- Watch Cardano's video.
- Open each tab, from left to right.
- Earn four badges to unlock The Sealed Letter.
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.
- A Cardan grille is a card with holes. Laid over a page, it shows only the letters or words of the secret. The card is the key.
- Students cannot click until they get it right. A missed question swaps in a new question, and three misses on a typed word bring a new grid.
- Make a grille is a building tab. Students punch holes, fill them in reading order, and write a short note around three secret words. The last one is a partner check, because the page cannot judge whether a note sounds natural.
- Grids and grilles are made at random, so neighbors will not have the same answers.
- The Sealed Letter is a team challenge. Its first lock uses a turning grille, which shows new letters at each quarter turn. Scratch paper helps.
- The Grille Workshop, a free builder, stays locked until the letter is open.
- Offline extension: this cipher is perfect for paper. Students cut holes in an index card, write a secret through them on graph paper, fill in decoy letters, and trade with a partner who has a matching card.
- History on this page: Girolamo Cardano described the grille in 1550. He was a doctor, a mathematician, and an inventor.
- Progress saves in the browser on this Chromebook only.
A card full of holes
In 1550, an Italian doctor and inventor named Girolamo Cardano had a clever idea. He cut a few holes in a stiff card. He called it a grille.
Lay the grille on top of a letter, and most of the words are covered up. Only a few words show through the holes. Those words are the secret message.
The card is the key
The sender and the reader each keep a grille. The two cards must be exactly the same, with every hole in the same place.
A snoop who finds the letter sees an ordinary note about a visit to an aunt. Nothing looks secret. Without the card, the snoop has no idea which words matter.
Each grille has a small gold star. The star always goes in the top row, so you know which way is up.
Letters work too
A grille can sit on a grid of letters instead of a page of words. Most of the letters are covered. The ones in the holes spell the secret.
Read the holes the way you read a book: left to right, starting with the top row.
How to write one
- Lay your grille on a blank page.
- Write the secret through the holes, in reading order.
- Lift the grille away.
- Fill every empty space with other letters. These are decoys. They hide the real letters in a crowd.
With a word grille, the last step is harder. The extra words have to turn the page into a letter that sounds normal.
Which card is the key?
A snoop searched a spy's desk and found a page of letters and a stack of grilles. Only one of the cards is the real key. The others are fakes, left there to fool anyone who looks.
Test each card on the page. The real one makes a real word.