Meet Knox. Press play to watch the video.
Your mission
Knox the mouse lives inside a wall. He hears people send secret messages with nothing but knocks. Today you will learn how they do it.
- Watch Knox's video.
- Open each tab, from left to right.
- Earn four badges to unlock The Secret Wall.
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.
- This lesson uses sound. Headphones help a lot. With sound off, the wall still flashes for every knock, so students can count by watching.
- Students cannot click until they get it right. A missed question swaps in a new question, and a missed word swaps in a new word.
- Words are picked at random, so neighbors will not have the same answers.
- The history here is real. American prisoners of war in Vietnam used this code to talk through cell walls. The page keeps that part short and gentle.
- The Secret Wall is a team challenge. It works best with a partner and a tap grid copied onto scratch paper.
- The Wall Phone, a free knock player and tap pad, stays locked until the wall is open. That way nobody can use it to skip the practice.
- Offline extension: partners sit back to back and knock short words on a desk. Row first, then column.
- This page pairs well with the Polybius square lesson, which uses the same 5 by 5 grid.
- Progress saves in the browser on this Chromebook only.
A code you can hear
Every cipher you have learned so far is written down. The tap code is different. You send it with sound.
During the Vietnam War, some American pilots were captured. They were locked in separate rooms and were not allowed to talk to each other. So they knocked on the walls. Knock by knock, they spelled out words.
You do not need paper or a pencil. You only need something to knock on.
The tap grid
The tap code uses a grid with 5 rows and 5 columns. You may remember it from the Polybius square. Every letter lives in one box.
To find a letter, say its row number first. Then say its column number. H is in row 2, column 3.
Row first, then column
To send a letter, knock its row number. Stop for a short pause. Then knock its column number.
H is knock, knock. Pause. Knock, knock, knock.
Wait for a longer pause before you start the next letter. The pauses tell your friend where one letter ends and the next one begins.
Where is K?
The alphabet has 26 letters, but the grid only has 25 boxes. So K moves in with C. To send K, you knock for C: 1 knock, pause, 3 knocks.
Your friend works out which letter you meant. If the knocks spell CITE, that is not a word they know. Swap in a K and it says KITE.
How to hear a code
- Press play and count the knocks.
- The first group of knocks is the row. The second group is the column.
- Find that box on the grid and click it.
- Play a letter as many times as you need.
Sound is off. Watch the wall flash for each knock instead.
How to tap a code
- Find the glowing letter on the grid.
- Press the knock button once for each row number.
- Stop and let the pause bar fill up. You will hear one beep.
- Press the knock button once for each column number.
- Stop again. Two beeps mean the letter was sent.
Keep a steady beat while you knock. If you wait too long in the middle of a group, the pause bar fills early.
Every knock costs time
Some letters are quick to send. A is in row 1, column 1, so it takes 1 plus 1. That is only 2 knocks.
Other letters are slow. Z is in row 5, column 5. That is 5 plus 5, or 10 knocks.
Knocking takes time, and a guard might hear it. So prisoners made their messages short. Instead of GOOD NIGHT, they tapped GN.