Propolis is a resinous mixture that bees collect from tree bus, sap flows, or other botanical sources. It is used as a sealant for unwanted open spaces in the hive. Propolis is used for small gaps (approximately 6.35 millimeters (0.3 in) or less), while larger spaces are usually filled with beeswax. Its color varies depending on its botanical source, the most common being dark brown. Propolis is sticky at and above room temperature. At lower temperatures it becomes hard and very brittle.