After a minor bike accident, many riders believe they need to buy a new helmet for fear that the crash may have compromised its safety. But now a smelly innovation will tell you if that purchase is really necessary. Researchers have made microcapsules that give off a pungent smell when they break open. These are put into the helmet's foam and crack during any serious impact that could damage the interior of the helmet. "If cracks form, smelly substances are released," Christof Koplin, a scientist at the Fraunhofer Institute for Mechanics of Materials in Germany, told the British newspaper The Times. The foam in helmets is designed to compress and crack in an accident, absorbing the impact before it reaches the rider's skull. Once it has compressed, it loses its impact-absorbing capability, so manufacturers advise riders to replace their headgear after a crash - even if the helmet appears undamaged.