Dye packs are those little bundles of surprise that banks use to help thwart robbers. Slipped in with the cash, the devices can be set off to explode dye all over the money, making the loot unusable and easy to spot.
What does it look like when one goes off? Here ya go:
(Via Action 19 News, reporting on a suspect who robbed a Stark County bank earlier this week and who is still on the loose.)