A common misconception is that the liquid drained from a brass instrument’s 'water key' (or spit valve) is saliva. In reality, it is mostly condensation. As the warm, moist air from the player’s lungs travels through the cold metal tubing of a trombone or trumpet, the water vapor condenses into liquid. If this water builds up, it creates a 'gurgling' sound that ruins the tone. The water key is a simple lever that opens a small hole at the lowest point of the instrument to allow gravity to drain the condensation.