First you must learn how to "Riffle Shuffle". Shuffle the cards at the corners so that you alternate, laying down a card from the left pile, then right, then left - etc.
The working of this trick is easy once you've practiced the riffle shuffle. I have found that if you riffle shuffle with a FEW mistakes (and I mean a FEW - do shuffle carefully) the trick
will still work.
You'll need to stack the deck on this trick. Separate the cards by colour. Place al
of the red cards on top of the deck and the blacks on the bottom.
Hand the deck to a spectator. Ask her/him to deal out (dealing properly) 4 equal piles. Now, have the spectator point to any two piles for you to shuffle together. Use the riffle shuffle. You now have 1 larger pile and 2 smaller piles.
Then riffle shuffle the remaining 2 small piles. Remark at how carefully and thoroughly these cards have been shuffled. You now have 2 piles of cards the same size. Announce that you
will shuffle these cards YET AGAIN. Ask the audience if they think these cards are well shuffled.
Now, here is an important part. Cut off about the top 1/3 of the deck and place it to your right. From the larger pile on your left cut off another third and place this in the middle. You now have 3 piles, roughly equal thirds.
Pick up the centre pile and remove a red card and a black card. Announce that these cards will act as magnets. Rub the red card on the left pile of cards and the black on the right pile. [I
prefer to slap the cards onto the piles, the kids jump and pay closer attention]. Pick up the red pile (the left), fan the cards and hold them out for all to see. Pick up the black pile (the right pile), fan them and hold them for all to see.
When doing this trick for kids under 10 you can really ham it up. Firstly, they are impressed that you can shuffle using the riffle shuffle because the average kid isn't ready to shuffle at all. Secondly they can't understand just how you did that! They may want you to repeat the trick. You can't unless you have a second stacked