You put together a jigsaw that has 17 robots, but then put it together again and now it only has 16. Or do the same with our Melting Snowman jigsaw drawn using our magic jigsaw creating algorithm. 

This trick is an example of a simple magic algorithm and a way of illustrating what an algorithm is. Have the class each follow the 3 simple steps of the algorithm:

1. Count the robots
2. Swap the pieces around in the way shown
3. Count the robots

The magic happens whether they know what they are doing or not, and even if they have no idea how it works. That is what a computer needs for its programs – instructions it can blindly follow and the right thing will happen even though it has no idea what it is doing (It can’t know as its just a lump of silicon!). That is what an algorithm is: instructions that if followed guarantee the right thing happens.

This trick also illustrates how apparently simple things can be too complex for our brains to take in. An important design principle is that interfaces should be clear and simple, not cluttered. It also illustrates aspects of computational thinking such as simplifying the problem and the importance of understanding people.

Turn this into an art activity by following the algorithm to draw your own pictures where objects in the picture magically appear and disappear (if you have the patience and attention to detail). We created the Melting Snowman version abovethat way.

For example as a christmas activity draw a version with christmas trees or snowmen where one disappears. See the cs4fn article below for the algorithm for drawing them.

Learn about:

• computational thinking
• human-computer interaction
• usability
• designing to prevent human error.

Resources

This session comes with the following resources:

• Activity Sheet including puzzle: The Teleporting Robot [PDF]
• Puzzle: The Teleporting Robot [PDF]
• Puzzle:  The Melting snowman [PDF]
• Magazine:  cs4fn issue 17: Machines Making Medicine Safer [PDF]
• cs4fn article: Algorithm for drawing your own magical picture [WEB PAGE]

This session is in collaboration with CHI+MED: an EPSRC funded project about making medical devices safer.

The trick is based on a very old puzzle at least one early version of which was by Sam Lloyd. See this selection of vanishing puzzles for some variations.  A very simple version of it appears in the Moscow Puzzles (puzzle 305) by Boris A. Kordemsky where a line is made to disappear which is where I first came across it as a schoolboy. A version is described by William Hooper in a book published in 1794 involving cutting up bank notes – as a way of cutting 9 notes in to 18 parts to make 10 bank notes. How to make money! See Multimedia Puzzles. Bank notes are now designed to stop the trick working on real notes.