Sharp pencils and water-filled sandwich bags demonstrate the nature of certain polymers.
Most materials commonly known as plastics are made of very long molecules called polymers. Chemists design polymers to have different characteristics, making them useful in a variety of situations. The material used to make sandwich and storage bags, polyethylene, consists of polymer molecules intertwined and linked to form a weblike matrix. That molecular web, with all its connections between molecules, gives polyethylene its characteristic resistance to tearing. That’s why the bag does not tear and holds tight against the sides of the pencil. No (or very few) leaks!