There are loads of ways to know where a card is in a deck, but some scientifically ambitious card sharps appear to have taken a very unusual approach. This strange story began when police in Berlin discovered unusually high amounts of radioactivity in a trash truck. They traced the material to a restaurant and discovered 13 playing cards that had been laced with iodine-125.
Thanks to a 60-day half-life, this particular strain of iodine stays radioactive for a long time. The detectives guessed that a player with a hidden detector would be able to identify the marked cards in a game, giving them an advantage over their competitors. Considering the restaurant where the cards were found didn’t have the permits for gambling, that’s still all conjecture.
Given the high risks of working with radioactive material, however, we’d recommend that you not try incorporating iodine-125 into your arsenal of card control tricks.