I read this in the late 80’s / early 90’s.  A girl discovers that her friends are using a cube like device that when activated sends them on adventures. The only adventure I remember is all of them being shrunk and swimming in a pool of spilled soda.   But with each use, they lose more of their knowledge and start using other words because they can’t remember the actual one-in one case saying “Hitchcock” instead of ketchup.   The girl finds out through her dreams(?) that the device has been sent from another world by a queen who’s already drained all of her subjects of all their knowledge.   I remember the cover was mainly black with the girl looking into a light.