Propolis or bee glue is a resinous mixture that honey bees produce by mixing saliva and beeswax with exudate gathered from tree buds, sap flows, or other botanical sources. It is used as a sealant for unwanted open spaces in the beehive. Propolis is used for small gaps (around 6 mm (1⁄4 in) or less), while gaps larger than the bee space (around 9 mm (3⁄8 in)) are usually filled with burr comb. Its color varies depending on its botanical source, with dark brown as the most common. Propolis is sticky above 19 °C (66 °F), while at lower temperatures, it becomes hard and brittle.