Show a pad, consisting of the cover and the image of a human mind. Ask a viewer to write a color, a name, a pin (or other) in the white space representing the mind. Then have him close the pad so that no one can read what he just wrote. Incredibly, you will be able to reveal to the public what the viewer had written a few moments before.
Simple and pocket-sized, you can always carry it with you.
It is printed on special material, you can use the pens for the erasable whiteboards.