Resin is a thick, sticky substance that oozes from certain trees, especially pine and fir trees, when their bark gets damaged. If you've ever touched a pine tree and gotten that gooey, amber-colored sap on your fingers that won't wash off easily, you've encountered resin. Trees produce resin to seal wounds in their bark, protecting themselves from insects and disease, much like how your body forms a scab over a cut.

Ancient peoples discovered that resin becomes hard when it dries, making it incredibly useful. The ancient Greeks used resin to waterproof their ships. Musicians still coat violin bows with hardened resin, called rosin, to help them grip the strings properly. Amber, the beautiful golden gemstone you might see in jewelry, is actually fossilized tree resin that hardened millions of years ago.

Today, scientists have created synthetic resins (made in laboratories rather than from trees) that are used to make plastics, adhesives, and coatings. When you see a shiny, hard plastic surface or use super glue, you're probably looking at a product made from synthetic resin. The word can also refer to any similar sticky, moldable substance that hardens over time.