Natural resins are organic substances produced by certain plants, particularly trees, as a defense mechanism against injury or infection. These sticky, viscous materials can harden when exposed to air, forming a protective barrier. Common sources of natural resins include pine trees, fir trees, and balsam trees. Natural resins have been used for various purposes throughout history, including in the production of varnishes, adhesives, and incense. They are also important in traditional medicine and are valued for their aromatic properties. Some well-known types of natural resins include amber and copal.