In many parts of the world, cash held in ATMs or in cash-in-transit vehicles is protected by so-called intelligent banknote neutralization technology . When a thief tries to force the ATM open, plastic packs filled with dye explode, spraying both the thief and the banknotes. These notes have now been demarcated as stolen. A shopkeeper may refuse to accept marked notes or may only accept them at a large discount to their face value. At this point, cash has ceased to be fungible. One banknote is not a perfect substitute for another. The dye used in banknote neutralization is often mixed with a taggant , a chemical marker that contains a unique combination of elements chosen from thirty or so rare earth metals. This ensures that a given block of cash is protected by a one-of-its-kind dye pack. So if the authorities apprehend the ink-stained thief with the marked cash, they can actually trace the stuff back to its original owner and return it. This incentivizes any would-be ATM thief to th...