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Dan writes in:

How are big closed cell polystyrene bocks made? I’ve seen them 12″ square and about 8′ long.

Expanded polystyrene foam (EPS or Styrofoam) is made from pellets of polystyrene, which is a plastic made from crude oil. The pellets are expanded in a steam chamber. You can find these pellets in this form inside most beanbag chairs. To make solid objects, they use steam molds that fill a chamber with the pellets, then steam fuse them into custom packaging, foam drinking cups, etc. Here are a few videos I found on the topic; one’s on Planet Green, the other one is a segment on Discovery’s How It’s Made:

Polystyrene is lightweight and good for protecting valuables in shipping, among other things, but it’s not biodegradable. It can, however be recycled. I found a video on Planet Green where they turn styrofoam packaging into moulding for your house. Neat! Also, don’t forget to check out the Wikipedia page on polystyrene foam.