What are the Nerf weapons made of?

NERF is made from a solid, spongy cellular material produced by the reaction of polyester with a diisocyanate while carbon dioxide is liberated by the reaction of a carboxyl with the isocyanate. Polyester resin reacts with a compound while CO₂ is simultaneously released by another reaction. It is this gas that creates open pockets within the polyurethane that, in turn, makes the material soft and light. (from Wikipedia)