CS Unplugged activity

Secret Messages in ASCII: lesson plan

A short teacher-led plan for Secret Messages in ASCII: every letter a computer stores is really a number. About 50 minutes. Leaving it for a sub? Use the box below.

For a sub: the 2-minute version

Skip everything below this box. Nothing to set up.

Optional, if you can project: type https://learn.porttack.com/go/ascii into the browser’s address bar (not a search box), press Enter, then use the right arrow key to step through. Stop at the slide titled On Your Sheet, then hand out the sheets. Unsure about a slide? Skip the slides: the sheet explains everything.

Hand out the worksheet, one per student. Names on top.
Say: “Computers store every letter as a number. Use the code table at the top of the sheet to decode the messages, then write your own for a friend.”
If the teacher left more puzzles, hand the next one to each student who finishes: a new set of secret messages.
Restless room? Let neighbors work in pairs. Talking about the puzzle is fine.
Collect every sheet, finished or not.

Before the sub day (teacher, or whoever preps the sub folder): print one copy of the worksheet per student, plus keep-going sheets from the New set page: set Copies to your class size and Sets per student to 2 (front and back). That covers a full period even for your fastest students.

Students will be able to

Explain that computers store letters, digits, and punctuation as numbers.
Decode and encode messages with the ASCII table.
Notice that capital and lowercase letters have different codes.

Before class

Print one worksheet per student.
Print keep-going sheets from the New set page: Copies = class size, Sets per student = 2 (front and back).
Optional: the slides.

The lesson

1. Hook (5 minutes)

Show the slides through 67 79 79 76 (or write 72 73 on the board). Ask: “If computers can only store numbers, how do they store a text message?” Decode 72 73 (HI) together, then let students try 67 79 79 76 (COOL) before you reveal it. The slides stop before the sheet’s own messages.

2. The sheet (20 minutes)

Students decode the first messages with the code table, then move on to lowercase, punctuation, and digits, then write their name in ASCII.

3. Secret messages for a friend (10 minutes)

Students write coded messages for a partner, swap, and decode. Keep it friendly: anything they write, the receiver will read out loud.

4. Keep going (as students finish)

Hand out keep-going sheets so neighbors get different messages.

5. Wrap-up (5 minutes)

Ask: “Why does ‘A’ have a different number from ‘a’?” (They’re different characters, so they need different codes. A computer can’t guess which you meant.)

Going further

Count the Dots first, if students haven’t seen binary yet.
Secret Messages in Binary writes the same codes as 0s and 1s.