Boxi Learn › Computing › Stage 2 · Models and Patterns
The card-flip trick: catching an error
CS-3 Any information can become numbersCS-4 Mistakes are normal — find themX-1 Guess, then check
What many people think
To notice a mistake in a message you have to remember the whole message.
The idea
Add one extra card to every row and column so each has an even number of black sides; then a single flipped card shows up as the one odd row crossing the one odd column.
Explain it
A friend says you can only spot a changed card if you remember the whole layout. Explain how the extra row and column let you find it, and why the trick fails when two cards are flipped.
Try at home
Use 25 coins or paper squares coloured on one side. Lay out 5 × 5 at random, then add a 6th card to each row and a 6th row so every row and column has an even number of coloured sides. Turn your back while someone flips one, then find it.