Polypropylene is a hard, light plastic. Factories use it for bottle caps, food boxes, cheap folding chairs, rope, car parts, and those slightly cloudy takeaway tubs that can often go in hot water or a microwave. Chemists mark it PP, and you will see “PP” stamped on the bottom of many containers. It is tougher than some other cheap plastics and does not like to soak up water. This is factory and recycling language; at home people just say “plastic.” Though actually, not every hard food box is polypropylene — the recycling mark is how you know. Do not mix it up with the white foam used for hot-soup boxes; that is usually a different plastic.