Polymers are large molecules made up of repeating units called monomers. They can be classified into two main types: natural polymers and synthetic polymers. Natural polymers, like cellulose and proteins, are found in nature and are essential for life. Synthetic polymers, such as plastic and nylon, are man-made and widely used in various applications due to their versatility and durability. Another way to categorize polymers is by their behavior when heated. Thermoplastics can be melted and reshaped multiple times, making them ideal for recycling. In contrast, thermosetting polymers harden permanently after being shaped, providing strong and heat-resistant materials for products like epoxy resins and rubber.