Use a pencil to quickly pierce the bag and water! What's going on? Most plastic bags are made of polyethylene, a polymer that is durable, flexible, and shaped like long strands. When the pencil is quickly pierced into the bag of water, the polyethylene molecules separate but don't break, forming a seal around the pencil. If the pencil is pierced into the bag before the water is put in, the water will leak! Try the experiment with a faceted pencil, a larger pencil, or a thicker plastic bag. What happens?