There is a tendency to utilize underground space for the less ornamental purposes of civilization; there is the Metropolitan Railway in London, for instance, there are new electric railways, there are subways, there are underground workrooms and restaurants, and they increase and multiply.
Polystyrene can be naturally transparent, but can be colored with colorants. Uses include protective packaging (such as packing peanuts and CD and DVD cases), containers (such as clamshells), lids, bottles, trays, tumblers, and disposable cutlery.
It is sometimes convenient to categorize packages by layer or function: primary, secondary, etc. 1  Primary packaging is the material that first envelops the product and holds it. 2  Secondary packaging is outside the primary packaging, and may be used to prevent pilferage or to group primary packages together.