Molecules and Polymers Here's the million-dollar question of the day: What are polymers? Hmm…Not sure? First you need to know what a molecule is. A molecule is the smallest part of something that is still that thing. "Huh?" you ask. Well, what's water? Besides wet, it's also known as H2O. H2O is a molecule-a water molecule made up of two hydrogen atoms and one oxygen atom. The glass of water you drink when you come in from playing outsides is just millions of H2O molecules floating around in your glass. "So what's a polymer?" Here's a quick Latin lesson: "Poly" means many and "mer" mean units or parts. So a polymer means…. You got it! It's a large molecule made up of many repeating parts or subunits. Water is not a polymer because it only has H2O.