The best number trick of all time.
This effect combines a little bit of mind reading and lot of impressive math wizardry. The Automatic Magic Square makes you look like a genius. Thankfully, the method only requires very basic elementary math skills.
They think of a number and immediately you start writing numbers down. It looks like their thoughts are guiding your pen to write numbers down. After you write many of the numbers down you ask for their number and in seconds you complete the magic square. You mention that while you did not write down their thought of number, something interesting has happened. You noticed that all of the numbers in each row equals their thought of number. Same goes for each column and the diagonal lines too. Not only that but you can even add the four corners and they add up to the thought of number. Whether it is mind reading, magic or math, the Automatic Magic Square is an impressive feat no matter how you add it up.