In a preferred embodiment of the invention, a closure for a container comprises at least two closure members. The relative position of the closure members with respect to each other is altered upon mating of the closure and container. The alteration of the relative position of the closure members may result in one or more of the members approaching the container more closely. The alteration may result in one or more of the members pressing against the container. The pressing may take place in such a way that exit paths for vapor from fluid in the container would require passage through the pressed surfaces.