Expanded Polystyrene in Bike Helmets
You may not recognize it, but you've probably used expanded polystyrene before, it's the same component that makes up a stryofoam cooler. It is also used in a slightly harder version to create the line the interior of bike helmets.
The reason that polystyrene is consistently chosen over other types of foam is the unique ability of polystyrene to absorb an impact and basically spread it out within the bike helmet. So an impact that affects one area of your head will actually be lessened by wearing a bike helmet that is made with an interior liner of polystyrene as any hit you take will be spread over the helmet as the foam absorbs the energy of the impact thus protecting your head. The downside is that because of this ability, your bike helmet will be good for one impact only. Even if you can not detect it with your naked eye, the interior foam has most likely cracked from absorption of the impact and your helmet will no longer be able to protect you adequately from falls and impacts.