I first encountered the Magic Number Squares in Arthur Mee's The Children's Encylopedia printed in the 1930's. I've seen variations of it since then, but still remember the awe I felt at impressing even grownups with this mind-reading numbers trick.
Instructions: Print the image below, and cut out the six rectangles. If you will be using this trick a lot, glue the page to cardboard before cutting out.
Ask someone to think of a number, then give them the Magic Number cards, and ask them to give back to you only those cards that contain the number that they have in mind.
Pretend to concentrate; you're thinking...it's coming to you...a bit fuzzy, but oh, yes -- The number is...!!!
What you do is quickly add up the numbers at the top right hand corner of the cards passed back to you -- they will reveal the number being thought of - like magic!