When a speeding object strikes a piece of plastic foam such as polystyrene, the work required to crush the walls of the millions of air cells in the foam slows the object down. That's why polystyrene is ideal for use in bicycle helmets and other protective gear.

Metals such as aluminum can also form foams -- and because of their greater rigidity, they could, in theory, dissipate as much energy as a polymer foam in a much thinner layer.