Plastic bags are made of materials called polymers. Polymers are made of small particles that bond together to form large chains. These materials are super useful because they are strong, flexible, and transparent so you can always see what is inside them. And, if you poke a hole in the bag with something sharp like a pencil, the particles can seal back around the holes as they try to retain their shape. As a result, the bag doesn’t even leak (at least much). Pretty amazing huh?