Effect

The 30-second “Countdown” theme is played (or similar) while the pupils quickly pass a Teddy round the class. When the music stops, Sameer is left holding Teddy. I ask Sameer to tell me the number of his house. He tells me that it is 46. Immediately I draw on the board the following square:

Quickly we add up each row: 46! And each column! The two diagonals as well!

The magic total 46 is obtained in every direction!

But then clever old Suraj and Emily have been adding in other ways. They point out that the corners add up to 46 too, and the 4 middle numbers! Before long the class has found that each corner 2x2 square totals 46, as well as the top/bottom half middle four.

Later it is noticed that
1 + 12 + 27 + 6 = 11 + 5 + 2 + 28 = 46, and
then Lateral Lisa pipes up with
1 + 11 + 28 + 6 = 12 + 2 + 5 + 27 = 46.

Method

In the square above, the four numbers in the twenties are the only numbers which are altered to make the trick work. I only need to learn this square:

When I first performed this trick in a classroom, it was back in the days of blackboards and chalk. I had used (cunningly, I thought) a pencil outline on the board which I was then planning to write over with the chalk during performance. Unfortunately for me, the graphite in the pencil was reflective enough to catch the sunlight and be perfectly visible to my audience, thus explaining my surprise as they called out the numbers before I had even chalked them in. Andrew Jeffrey has subsequently given me the far more professional and useful tip that this square could be stuck on the barrel of the whiteboard pen. It could even be memorised!