Glues are liquid substances that hold materials together and prevent separation after drying. They often form a permanent bond. Traditionally, glues have been derived from natural products such as animal hides, but synthetic formulas have also become very common. They are formulated for bonding materials in specific applications, such as wood joinery or shoe repair. All-purpose glues include products capable of bonding materials that other glues may not adhere to, such as plastic or glass.