Kjartan Poskitt at the National Young Mathematician Awards in Cambridge University

The Trick:

1/ Ask a friend to think of a secret number between 1-16. Then...

2/ Lie the four special cards out.

3/ Ask your friend to make sure the secret number is the right way up on each card. Here you'll see the two cards marked with a red dot have been rotated.

4/ Stack the cards up on top of each other...

5/ ... it doesn't matter what order the odd-shaped cards are piled in so long as you don't rotate them.

6/ Finally the square card goes on top.

7/ Turn the whole pile of cards over, and your friend's secret number will appear in the hole!

Make your own set of cards!

You can cut out your own set of cards from this sheet.

Click on this little image to open up the big version and print it out.

Otherwise you can print off this Predictor Cards word doc which also gives you instructions for doing the trick.