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CBSE Computational Thinking & AI · 2026–27

Class 3 · Computational Thinking
Chapter 1 Teaching Pack

What’s in a Name? — a practical introduction to letter-shift ciphers, ready for the Maths classroom.

Complete pack Lesson plan Worksheet + answer key Unplugged activity Project rubric Evidence record

Where this chapter sits

This is the opening Computational Thinking chapter for Class 3. It sits inside the Maths book sequence, so the Maths teacher can teach it alongside work on order and number moves. Students apply the same counted move to letters instead of numbers.

#	Chapter	Place in the year	Learning focus	Suggested
1	What’s in a Name?	Opening chapter; before later shape and reasoning work	Read a shift table, use a key, encode forwards, decode backwards, and explain why the key matters	5 periods

Assessment note Watch the process, not only the final word. A student who shifts every letter in the same direction and checks by reversing the shift has understood the rule. Keep the worksheet, project rubric and a short observation record as evidence.

Lesson plan · 5 periods

Period 1 — A rule changes the message

Write CAT on the board. Under it write DBU. Ask, “What happened to every letter?” Give quiet thinking time before taking answers. Students usually notice C→D first; insist that they check A→B and T→U before accepting the rule.

Name the parts only after the pattern is visible: the readable words are the original message, the changed letters are the coded message, the changing rule is a cipher, and the number of places moved is the key. This classroom shift cipher is often called a Caesar cipher.

Safety boundary Use made-up or classroom words only. Students must not write real passwords, PINs, addresses or other private information. A small letter shift is a thinking activity, not secure protection for important information.

Period 2 — Build the shift table

Use one half of the table at a time. Find a letter in the Original column, then read across to the chosen key column. The alphabet loops: after Z comes A.

Original	Key 1	Key 2	Key 3	Original	Key 1	Key 2	Key 3
A	B	C	D	N	O	P	Q
B	C	D	E	O	P	Q	R
C	D	E	F	P	Q	R	S
D	E	F	G	Q	R	S	T
E	F	G	H	R	S	T	U
F	G	H	I	S	T	U	V
G	H	I	J	T	U	V	W
H	I	J	K	U	V	W	X
I	J	K	L	V	W	X	Y
J	K	L	M	W	X	Y	Z
K	L	M	N	X	Y	Z	A
L	M	N	O	Y	Z	A	B
M	N	O	P	Z	A	B	C

Model BAD → CBE with key 1. Point to one column for each letter. Do not let students count from the letter as “one”; one move from B lands on C.

Period 3 — Encode with one steady key

Move from the Original column across to the chosen key column. Work one letter at a time and leave spaces unchanged. Model MATH → OCVJ with key 2, then have pairs check BOOK → ERRN with key 3.

The habit to teach Write the key beside the work before moving any letter. Most errors come from changing the key halfway or counting the starting letter as the first move.

Period 4 — Decode by reversing the move

To decode, find the coded letter in the chosen key column and read back to the Original column. This is the same table used in reverse, not a new rule. Model UWP → SUN with key 2. Then let students decode ILVK with key 3; the answer is FISH.

Ask students to check their own code: encode the word, cover the original, and decode the code. If the same word does not return, at least one letter moved incorrectly.

Period 5 — Apply, explain and assess

Run the unplugged activity in Section 05, then use Questions 16–18 from the worksheet as the individual check. Question 18 matters most: it shows whether a student understands that a coded message without its key can have more than one possible original.

Worksheet

Name: Class & Section: Date:

Use the shift table in Section 02. For encoding, move forward. For decoding, reverse the move.

A · Read the rule

1What tells you how many places to move each letter?

(a) The key(b) The page number(c) The word length(d) The date

2With key 1, A becomes:

(a) Z(b) B(c) C(d) D

3With key 1, Z becomes:

(a) A(b) B(c) Y(d) Z

4With key 2, C becomes:

(a) A(b) D(c) E(d) F

5With key 3, T becomes:

(a) Q(b) U(c) V(d) W

6To decode the first letter of UWP with key 2, U must move:

(a) forward 2 to W(b) backward 2 to S(c) forward 1 to V(d) backward 1 to T

B · Encode

7Encode CAT with key 1.

8Encode SUN with key 2.

9Encode MAP with key 3.

10Encode ZOO with key 1.

11Encode SCHOOL with key 2.

C · Decode

12Decode DBU with key 1.

13Decode UWP with key 2.

14Decode ILVK with key 3.

15Decode UFTU with key 1.

D · Check and reason

16Nina encoded RAIN with key 2 and wrote TCJP. One coded letter is wrong. Circle it and write the full correct code.

17MAP became OCR. What key was used? Show one letter pair that proves it.

18A card says BQQ, but its key is missing. Leela says the original message must be ZOO. Is she correct? Decode BQQ with key 1 and with key 2 to explain.

Answer key & teaching notes

Q	Answer	What to watch for
1	(a) The key	Students may say “rule”. Ask for the number that fixes the size of the move.
2	(b) B	One move starts after A; A itself is not counted as the first move.
3	(a) A	This checks wrap-around. The alphabet continues at A after Z.
4	(c) E	Have the student say the two moves aloud: C→D→E.
5	(d) W	Check for three moves: T→U→V→W.
6	(b) backward 2 to S	Encoding goes forward; decoding reverses that direction.
7	DBU	Check each pair separately: C→D, A→B, T→U.
8	UWP	S→U, U→W, N→P. A common error is to use key 1 after the first letter.
9	PDS	M→P, A→D, P→S. Students should use the same key for all three.
10	APP	Z must wrap to A; both O letters must produce the same coded letter.
11	UEJQQN	S→U, C→E, H→J, O→Q, O→Q, L→N. Repeated O must give repeated Q.
12	CAT	Read from the Key 1 column back to Original: D→C, B→A, U→T.
13	SUN	U→S, W→U, P→N. Watch for students who move forward again.
14	FISH	I→F, L→I, V→S, K→H. Each letter moves back three.
15	TEST	U→T, F→E, T→S, U→T. The repeated U gives repeated T.
16	TCKP; J is wrong	R→T, A→C, I→K, N→P. The student must locate the error, not recode only the wrong letter.
17	Key 2; M→O, A→C or P→R	Any one correct pair proves two forward moves. Do not accept “2” with no letter pair because the question asks for evidence.
18	No. Key 1 gives APP; key 2 gives ZOO.	This is the discriminating question. The same code can produce different originals when a different key is tried, so the missing key matters. Check B→Z when moving back two through A.

Marking the reasoning For Question 18, full credit needs both decodings and the conclusion that Leela cannot be certain without the key. A student who recognises ZOO but does not test another key has guessed a likely word rather than explained the cipher.

Unplugged activity · “Cipher Post Office”

Period 5 · 35 minutes · No devices, no internet, no lab required · Planned for 40+ students.

What you need

The shift table in this pack, the blackboard and chalk, one sheet of used-on-one-side paper per group, and pencils. Make eight groups of about five students. Pair the groups: 1 with 2, 3 with 4, 5 with 6, and 7 with 8.

Set the room before starting

Write these roles on the board: word chooser, encoder, checker, post carrier, decoder. Each child has one job, which prevents one confident student from doing the whole task. Write the allowed keys—1, 2 and 3—and the rule “classroom words only”.

How it runs

Each group chooses one classroom-safe word of three to six letters and one key. The word chooser writes both where the paired group cannot see them.
The encoder uses the shift table. The checker follows every letter down its column and signs the slip only when the code is consistent.
The post carrier delivers the coded word and the key to the paired group. Students stay seated except for the eight carriers.
The decoder reverses the shift. The paired groups compare the recovered word with the original. If they differ, both groups trace the letters until they find the exact move that failed.
Rotate roles and repeat with a new word. On the third round, send the code first and hold back the key for one minute. Let the receiving group try two keys before the carrier supplies the real one.

The moment the concept lands Ask, “Why was the third message harder before the key arrived?” The useful answer is not merely “we did not know”. Students should say that different keys give different possible original letters, so the receiver needs the sender’s rule.

Practical notes Keep messages to six letters so all groups finish. If a group struggles, give it BOOK with key 1; the code is CPPL. Do not use names, passwords or private details. Collect one checked slip from each group as activity evidence.

Project brief & rubric

This small project checks whether students can make, use and explain one consistent cipher. It can be completed individually or in pairs using paper, pencil and a ruler.

Brief given to students “My Secret-Message Kit” — Choose key 1, 2 or 3. On one sheet, make a complete Original row and Shift row from A to Z. Write two classroom-safe original messages of three to six letters and encode both. Give one code and the key to a partner, ask them to decode it, and record whether it returned to the original. Finish with two sentences: one explaining how to encode and one explaining how to decode.

Criterion	4 — Exceeds	3 — Meets	2 — Approaching	1 — Beginning
Shift rule & table	All 26 mappings and wrap-around are correct	One mapping error at most; key is clear	Rule is visible but several mappings are wrong	No consistent key or usable table
Encoding accuracy	Both messages are fully correct and independently checked	Both complete with no more than one letter error	At least half the letters follow the chosen key	Codes do not follow one rule
Decoding & check	Both codes return exactly to the originals; any correction is shown	Partner check returns one message exactly	A decoding attempt is shown but is incomplete or incorrect	No decoding check
Explanation & presentation	Clearly explains forward and backward moves; work is easy to follow	Names the key, gives both explanations and is readable	Key or one explanation is unclear	Explanation is missing or work is incomplete

Suggested: 16 marks total. Record the four criterion scores so the feedback shows whether the student needs help with the rule, accuracy, checking or explanation.

Evidence record

Keep one completed page for each class and section. Attach or file the named samples with it.

Field	Record
School
Class & section
Chapter taught	CT Ch. 1 — What’s in a Name?
Periods used
Dates
Teacher
Activity conducted	Cipher Post Office (unplugged shift-cipher activity)
Assessment used	Worksheet and “My Secret-Message Kit” project, rubric-scored
Students present
Students assessed
Samples retained	☐ 3 marked projects ☐ 3 completed worksheets ☐ Group cipher slips
Common error noticed
Next teaching step
Teacher’s signature

Quick class summary Students who can encode with one key: ____ / ____ Students who can decode by reversing it: ____ / ____ Students needing another table-guided practice: ____ / ____