Parity magic

Steps

1. Tell the class you have a magic trick. Ask a helper to lay out a 5 by 5 square of cards, some black and some white, in any pattern.
2. Say you will add one more row and one more column. Add cards so that every row and every column has an even number of black cards.
3. Now you have a 6 by 6 square. Ask the class to close their eyes while the helper flips exactly one card.
4. Look at each row and column. Find the row that has an odd number of black cards, and the column that has an odd number.
5. The card where that row and column cross is the one that was flipped. Flip it back.
6. Let pairs try the trick on their own.

Talk about it

• Why does the trick always work?
• Where might a computer need to find mistakes, like a scratched disc or a QR code?