Magic Squares, Any Total

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!
I don’t go on to reveal this trick to my students for two reasons. Firstly it is in the
working repertoire of several professional magicians (I first saw it done by Paul
Daniels), but secondly and more importantly, the impact of the apparently endless
totals is immediately lost. I prefer to leave them with that sense of wonder.