A dye pack is a radio-controlled device used by banks to foil a bank robbery by causing stolen cash to be permanently marked with dye shortly after a robbery. They were invented in the United States in Georgia, in 1965. In most cases, a dye pack is placed in a hollowed-out space within a stack of banknotes, usually $10 or $20 bills. This stack of bills looks and feels identical to a real one, but usually subtly marked in a way that is only privy to selected bank employees. The development of flexible dye packs makes it virtually impossible to detect by persons handling the stack.