Modern helmets are constructed from plastics, often reinforced with kevlar or carbon fiber. They generally have fabric and foam interiors for both comfort and protection. Motorcycle helmets are generally designed to break in a crash (thus expending the energy otherwise destined for the wearer's skull), so they provide little or no protection after their first impact. Note that impacts may, of course, come from things other than crashing, such a dropping a helmet, and may not cause any externally visible damage. For the best protection, helmets should be replaced after any impact, and every three or so years even if no impact is known to have occurred.