Polystyrene is a plastic that can be light and foamy, or hard and clear. The foamy kind is used for takeaway boxes, packing chips, and cheap coolers, because the air inside it slows heat and cold from moving through. The hard kind was used for things like old CD cases. In English shops the foam is often called Styrofoam, though that name belongs to one company; the material itself is polystyrene. Chemists write PS on recycling marks. It is light, cheap, and breaks easily, and it is hard to recycle well. Though actually, the white foam and the clear hard plastic are the same family of material, just made in different ways — students are often surprised by that.