polypropylene, polystyrene...ahhhh, what's the difference?

From World...

Instead of banning Styrofoam coolers from a number of Show Me State rivers, a law that took effect in August will ban Tupperware from Missouri waterways....The law bans polypropylene [but they]...were seeking to ban expanded polystyrene...So for now, beer drinkers can keep their coolers on the river, but they will risk a year in jail if they bring along leftovers in Tupperware...