Cas reaches over with a sigh and places his hand just barely over Dean's mouth, palm against his lips, before withdrawing and lacing his fingers over his stomach again. "A common human misconception is that space, as a vacuum, has no sound. This isn't true; objects within space don't produce a sound humans can hear due to the lack of air molecules. However, angels can hear waves, and space contains every conceivable kind of wave ever to exist."