You've probably heard of bank robbers getting caught because of a "dye pack" hidden in their stolen cash. But what exactly is a "dye pack"?

In a typical bank heist, the thief hands the teller a note with demands, requesting that money be placed in a bag or container. The dye pack was created as a method to neutralize the robbery non-violently by permanently marking the stolen money with a bright red stain, signaling to everyone that the money is not legitimate.

The dye pack used in more than 75 percent of U.S. banks is known as the "SecurityPac," manufactured by ICI Security Systems. This device contains a bundle of genuine bills, usually $10 or $20 bills, with the dye packet inserted in the middle. Previously, the device was encased in rigid plastic, making it detectable. Now, advances in technology allow the dye to be encased in a slim, flexible material, making it almost indistinguishable from a regular money stack.

Tellers always keep a supply of these dye packs nearby. To activate a pack, it's placed in 'safe' mode using a special magnetic plate. During a robbery, the teller discreetly places one of the dye packs into the thief's bag. The pack remains inert while inside the bank, but contains a tiny radio receiver. When the pack passes through the door, it picks up a signal from a transmitter located near the entrance. The dye pack activates, often with a delay of 10 seconds or more, ensuring the thief has already fled the scene before the dye explodes.

When the dye pack detonates, it releases a burst of red smoke, red dye (1-methylamino-anthraquinone), and occasionally, tear gas. This chemical reaction causes the device to burn at around 400 degrees Fahrenheit (204 degrees Celsius), making it nearly impossible to remove from the bag. (Details about the chemical process remain 'classified.') The explosion often forces the thief to discard the bag, allowing the bank to recover its funds. Moreover, the red dye typically stains the thief's clothing and/or hands, making it easy to identify the culprit.

As of now, the "SecurityPac" has been instrumental in recovering nearly $20 million and capturing approximately 2,500 criminals.