A material used to hold two surfaces together is called an adhesive. An adhesive functions by wetting the two surfaces, adhering to them, developing and maintaining strength after application, and remaining stable. Engineering adhesives are a type of adhesives that are widely used to bond and seal parts of automobiles, electric appliances, electronics, office machines, medical equipment, building materials, furniture, rolling stocks, optical equipment, and glass ceramics. These adhesives are mostly made up of acrylics, epoxies, and urethanes. Based on epoxy, polyurethane, acrylic, and other technologies, engineering adhesives are used to bond materials such as steel, aluminum, engineering plastics, and composites as a substitution for traditional joining techniques. Major types of engineering adhesives are epoxy (one part, two part, and films), acrylic, anaerobic, cyanoacrylate, urethane, and silicone.