Developers have come up with a bicycle helmet that emits a terrible "stink" if it has cracked and is no longer protective. This way - riders know definitively that the helmet is no longer protective.
Some helmets will crack on the inside - but not appear to be cracked to the wearer. In most cases, riders will throw out helmets when in doubt - to err on the side of safety. But, this is wasteful for those that dispose of helmets that drop -but are not damaged.
In this case, the helmet emits a stink so bad that there is no doubt that the helmet is no longer protective. Quite ingenious, particularly when "inexpensive" helmets run a minimum of $50 a piece.