Pick a number from the following grid.
Now pick another number. The number has to be in another column and another row. So if you picked 10 (third row, second column), you cannot pick a number in the same row (9, 11, 12), or in the same column (2, 6, 14). You might want to cross out the disallowed numbers.
Pick a third number, again in a different column and different row from the previous two numbers.
Pick a fourth number, again so it’s not in the same row or column as any previous number.
Now I want you to add up the four numbers.
Even though you freely selected four numbers, I exactly can predict a pattern in the numbers you picked.
I am confident the sum of your numbers is 34.
The puzzle is: why does this trick work?
