A climbing and mountaineering helmet made entirely out of foam? Sounds like a terrible idea, right? That's what we thought too. But Petzl's Sirocco is made out of the same stuff (expanded polypropylene) that the auto industry uses to reinforce car bumpers. The foam has an exceptionally high rate of energy absorption and structural strength, and is much more durable than the typical EPS foam, i.e. Styrofoam, that you find inside many helmets. The stuff is so tough that Petzl decided to not put a reinforcing layer over it, and the result is one of the lightest climbing helmets in the world. At just 5.8 ounces, it weighs up to half as much as a typical climbing lid.