Best card trick ever!

Back in high school, a teacher decided to reward the class by letting students go home early. The catch - he would present a card trick with the help of an assistant, and we had to figure out how he did it. Anyone who could whisper the solution in his ear could go home right away.

Here's what he did. He laid a 3 by 3 grid of playing cards, face up on the table. His assistant was already prepped for the trick. He then told us that he was able to teach people how to tune in to telepathic messages and he would prove it by allowing anyone to point to one of the 9 cards on the table and he would communicate the chosen card silently to the assistant so that they would know which card it was. Once someone pointed to their chosen card while the assistant looked away, he would then call the assistant over and point to various cards, asking if it was the chosen card. The assistant simply answered with a yes or no after each point and was correct every time. Theories started flying in earnest. It was the 3rd card pointed to! Proven wrong on the next demonstration. It was a look, or a facial expression, or something..... but in most cases, the students were wrong. The actual solution was much simpler.

Both the teacher and the assistant had to imagine the 3 by 3 grid of cards not only on the table, but duplicated on each card. When a card was chosen by someone, the teacher indicated (to the assistant) on the first point where in the grid the chosen card was in the grid (the card circled in the picture) by pointing to that imaginary grid spot on a card (the small circle on the example card). So if the chosen card was in the top right corner of the grid as shown in the picture, you simply pointed to the top right corner of the first card you pointed to. That was the tell. If the assistant paid close attention, the indication could be quick and almost imperceptible. The key is to mix it up - don't always point to the correct card on the same number of tries, if you catch my drift. In another example, if the chosen card in this grid was the Ace of Hearts and I pointed first to the centre of the Ace of Hearts asking, "Is this the chosen card?".... the correct answer would be 'yes'.

What I enjoy about this trick even today, is that once someone else in the crowd has it figured out, you can even invite them to be the recipient of your telepathic thoughts (card guesser) and they will magically get it right too.