Quick card flip magic

Mathematics: Numeracy

Every item of data that we store and transmit has extra bits added to it to prevent errors. Can you find the error and correct it?


At least 36, or even 100 or more cards that are black on one side and white on the other, about 20mm by 20mm (or any two colours that are easily distinguished) A clear space on the floor or a table that the students can stand around


Select a ‘magic master’. This is the person who is in control of the game. This person will change at the end of each round.


  1. The magic master sets up the cards on the floor, including parity cards in the last row and column, using someone from the group as their assistant. The grid can be any size; it should be at least 6 by 6, although it can be increased up to 10 by 10 or more to make the challenge harder. The grid doesn't have to be square (e.g. 9 by 8 is fine), but the effect is greatest when it is close to square.

  2. The magic master asks everyone to close their eyes and turn away, except for the magic master and the assistant.

  3. The magic master asks the assistant to choose a card, place a counter or a mark under where the card goes and flip it over.

  4. Once this has been done, the magic master calls out (quietly)…” let the magic begin” and presses the timer. When the other students hear this they turn around and try to find the ‘error’.

  5. As soon as they spot the error, they put their finger on their nose.

  6. The magic master stops the timer and asks the first person who put their finger on their nose to show where the flipped card was.

  7. The student points to the flipped card, checks if they are correct by flipping the card over.

  8. If they are correct, that person explains how they worked out which was the flipped card.

  9. The magic master records the person's name who won that round and the time.

  10. Did they beat the previous time?

  11. If they did beat the previous time, that person becomes the assistant. The person who won that round stays the assistant until their time is beaten.