A stopper which engages a neck of a container. The stopper includes a circular bottom section and a head section which rises above the container and encompasses a slide. The rim of the head section has a maximum diameter that does not exceed the external diameter of the opening of the container. The slide can be pushed against the force of a spring device in one direction and, when released, returns to its retracted initial position as a result of the spring force. Opposing sides of the slide each end in circumferential sections. A first one of the circumferential sections forming an outer surface for receiving a first applied force for advancing the slide and causing the second one of the circumferential sections to protrude from the stopper. The second one of the circumferential sections receiving a second different applied force to remove the stopper from the container.