Propolis is a resinous mixture that honey bees collect from tree buds, 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 millimeters (0.2 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 (20° Celsius). At lower temperatures it becomes hard and very brittle.Propolis is marketed by health food stores as a traditional medicine, and for its claimed beneficial effect on human health. Natural medicine practitioners use propolis for the relief of various conditions, including inflammations, viral diseases, ulcers, superficial burns or scalds. Propolis is also believed to promote heart health, strengthen the immune system and reduce the chances of cataracts. Old beekeepers recommend a piece of propolis kept in the mouth as a remedy for a sore throat.Some of these claims are being clinically investigated and several studies are published in the biomedical literature.