Have someone mentally think of any letter of the alphabet; A-Z. By having him simply sort through the 7 wood cards and selecting only those cards that contain the chosen letter (turning them upside down so that you have no clues), you will quickly be able to tell them the exact letter they were thinking of.
This is an idea from Maurice Kraitchik's 1942 book on titled Mathematical Recreations. They can be great fun and are quite astounding in how they work.
They are each 4" x 1.3" made from 1/8" thick alder. They are precision made in our Florida shop and the text is all deeply laser engraved.