1. Computational Thinking Activities The Magic of Computer Science
Aram, Job, Grant
The Magic of Computer Science
Materials from CS4FN. The Magic of Computer Science. Card Tricks Special . Peter McOwan and Paul Curzon. Queen Mary University of London.

2. The 21 Card Trick
Card trick activity demonstrates Algorithmic thinking.
“The point about an algorithm is that if you follow
its instructions exactly, you are guaranteed
to achieve what you are trying to do...if the
algorithm is correct” - CS4FN
CS4FN also link this activity to testing :
“How could we make sure our algorithm is correct
and our trick does work? Well we could do the
trick lots of times and check it works every time” – CS4FN

3. Activity 1. 21 Card Trick – With Cards KS3
Steps:
Show the class the trick.
With the algorithm teach a group of pupils the trick.
This initial group of pupils then show their own groups the trick.
They then supervise the groups learning the algorithm.
Learning Objective :
Understand that by following an algorithm, exactly, the outcome is
guaranteed and that not following an algorithm correctly will lead to an error.

4. The high level algorithm
Deal 21 cards in 3 columns of 7
Ask the audience member to pick a card but do not tell the magician
Ask the audience member to identify the columns containing the card
Place the chosen column of cards between the other two
Repeat the process for 4 deals
The chosen card is now in the middle of the middle column

5. Problems and Misconceptions.
It is expected that the pupils will make mistakes when following the algorithm.
This will emphasise the importance of precision.
Scaffolding and a range of learners.
Use fewer cards i.e. columns of 5.
Mixed ability groups. Support and help during exercise.
Pre Requisite Skill and Knowledge. ?
Resources.
Cards and Algorithm.
Links to curriculum and computational thinking progression (CAS)
Understands that computers need precise instructions.(AL)
Detects and corrects errors i.e. debugging, in algorithms.(AL)

6. Activity 2. 21 Card Trick – With Python More able KS3 / KS4
Steps:
Remind class/ introduce the class to the trick.
Provide High level algorithm from task 1
Decompose this into more detailed algorithm
Use more detailed algorithm to program algorithm (Python)
Learning Objectives :
Further understand that by following an algorithm, exactly, the outcome is
guaranteed and that not following an algorithm correctly will lead to an error.
Understand that splitting an algorithm into smaller parts make it easier
to understand (and convert into code) – Decomposition.

7. Problems and Misconceptions.
Again it is expected that the pupils will make mistakes when programming the algorithm
emphasising the importance of precision and introducing / emphasising debugging.
Scaffolding and a range of learners.
Partially completed code.
Support and help during exercise.
Pre Requisite Skill and Knowledge.
Python (Lists, Loops, Input/Output, Variables)
Resources.
PCs, Python Environment,
Links to curriculum and computational thinking progression (CAS)
Understands that computers need precise instructions.(AL)
Detects and corrects errors i.e. debugging, in algorithms.(AL)
Understands that algorithms are implemented on digital devices as programs. (AL)
Designs simple algorithms using loops, and selection i.e. if statements.(AL)