1. Randomly check the boxes in a 5x5 grid.
2. Add a 6th row and column of unchecked boxes.
3. Check the boxes in the new row and column such that there are an even number of boxes checked in each row and column.
4. Have an audience member toggle a box while you close your eyes.
5. Magically deduce which box was toggled by finding the row and column with an odd number of checked boxes. Their intersection was flipped.