Mathematical card tricks are the best ones to learn, because no sleight-of-hand is required. A hat tip to Mark Frauenfelder for creating this video to illustrate the methodology of a clever trick posted at Greg Ross' Futility Closet.
Readers who are math enthusiasts may be able to explain to the rest of us how/why this trick works. I'm satisfied to accept it as magic.
Reposted from 2013 so I can relearn this trick before our upcoming family reunion.