Here is a handout for some basic
math tricks I did at an elementary school.
I started by reading a student's mind: she picked a number, and
wrote it on the board (outside my field of view), and then did
various operations as directed, and ended up with 4 (still
outside my view but always gives 4). I said:
Think of a state that starts with the corresponding letter; if
you have "1", you might use Alabama or Arizona, and if you have
"3" you might use Connecticut or California, like that. Now,
stand right in front of me and think of your state...
Is it Delaware?
They were amazed. When I explained they had to get "4", they
were still amazed: how did I know Delaware? So I asked them how
many states start with a "D". The trick works because you
*think* you have a choice about the math result (since you
picked the numbers), and you *think* you have a choice about
what state you choose, but I knew the answer was Delaware before
The reason the volunteer uses the board is that the class can
watch the math and catch if any arithmetical mistakes occur.
Then I did a few of the others from the handout, using "M" and
"D" and such, instead of numbers, on the board, to show how you
can see the inner workings of such a trick. Finally, I had them
group up and make up some themselves.