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Truth or Lie

Age 7 to 11
Challenge Level

Truth or Lie

Watch the video below:

If you can't access YouTube, here is a direct link to the video: TruthOrLie.mp4 

What do you notice?
Do you have any questions?

Watch it again.

Perhaps some of your questions have been answered.  Or you might have thought of new questions.

Using a pack of cards, can you work out how to perform the trick yourself? 

Why is this maths and not magic?


This trick, sometimes called The Nine Card Problem, was invented by Jim Steinmeyer.

Why do this problem?

Card tricks tap into children's natural curiosity and can provide the motivation for exploring the underlying mathematics in order to unpick how they are done.  This particular trick is surprising and intriguing, and will require learners to be resilient as well as methodical.

Possible approach

Play the video without interruption and ask the class what they notice.  They may also have questions but rather than share ideas or answer questions at this stage, play the video once more and invite everyone to watch again. Some of the children's questions might have been answered by seeing the clip for a second time, but you could now gather 'noticings' and questions, writing them up for all to see rather than commenting/answering yourself.  It may be that other learners can offer further insight so that some of the questions are answered.

At this point, emphasise that this trick isn't 'magic', it is maths, and ask learners in pairs to try to replicate it. You can leave the video playing on loop so that children can tune in or out at any point.  Suggest that children take it in turns in their pairs to do the trick, while the other watches carefully so they work together to try to figure it out.  

As they work, look out for those pairs who seem to have found useful strategies, for example a way of recording and/or a way of keeping track of the chosen card. You could facilitate a mini plenary to give time for some pairs to share their ways of working, which may help those who are struggling to make progress.

This could be a simmering activity, so that you leave time for learners to work on the task over a period of a few days, dedicating an area of your 'working wall' to sharing noticings.

A final plenary might involve learners recreating the trick themselves and explaining what they have found out. You might expect them to be able to say something about the position of the chosen card, but not necessarily why this always happens.

Key questions

Could you somehow keep track of the card that was looked at?
Does how you return the cards to the pile when spelling out the card name, matter?
How could you record/share what is going on?

Possible support

You could suggest that learners use a sticky note or something similar to keep track of the chosen card, or turn face up (as opposed to all the other cards being face down) when collecting up the cards for the first time. Knowing that the trick works whether you use the word 'magic' or the word 'maths' might give some a clue.

Possible extension

Invite learners to explore one or more of the following questions:

Does it matter if you lie and choose a card that isn't within the nine cards used?
If you had more than nine cards, what would happen?
Can you explain why the trick always works?