A polymer is a large molecule made by linking together many smaller, repeating units, like connecting thousands of identical LEGO bricks into a long chain.

Polymers are everywhere in your life. Your DNA is a polymer, with millions of small units carrying genetic information linked together. Proteins in your muscles are polymers made from chains of amino acids. Cellulose, which gives plants their structure, is a polymer made from sugar molecules.

Scientists have also learned to create synthetic polymers in laboratories. Plastic is perhaps the most familiar: polyethylene (used in shopping bags), polystyrene (used in foam cups), and nylon (used in clothing and rope) are all synthetic polymers. Rubber, whether natural or synthetic, is also a polymer.

The key to understanding polymers is repetition. Just as a long train is made by connecting many identical cars, or a pearl necklace is made by stringing many pearls together, a polymer gains its properties from linking many small units (monomers) into chains. These chains can be incredibly long, sometimes containing millions of units. The length and arrangement of these chains help determine whether a polymer is flexible like plastic wrap, strong like nylon rope, or stretchy like a rubber band.