Abstract: A closure system is disclosed for closing a container opening. The closure system includes a stopper member, a cage member for mounting to the container, and a spring member. The stopper member has a plug portion that tightly fits into the container opening and a cover portion that abuts a boundary surface adjacent to the container opening container. The stopper member plug portion is fitted into the container opening, the stopper member cover portion abuts the boundary surface of the container, and the cage member is mounted to the container to hold the stopper member, when the closure system is in an assembled state. The cage and spring members are arranged to position the spring member between the stopper member cover portion and the cage member such that the spring member pushes the cage member away from the cover portion when the closure system is in the assembled state.