Helmets vary in size, type, and accessories on the basis of the user's activity (Figure 192-1). They consist of a hard plastic, polycarbonate, and/or fiberglass shell over either a layer of foam covered by material, inflatable air bladders, or both. Motorcyclists and racers wear full-face helmets with or without retractable or removable visors. Football, lacrosse, and hockey players use open-faced helmets. These may have clear visors and/or face cages whose bases are screwed into the helmets. Bicyclists, kayakers, roller bladers, skateboarders, and skaters wear simple skull helmets. These helmets cover the top of the skull like a hat and have a strap that is snapped or clipped under the chin to maintain the helmet in position.