A stopper, configured to engage with an annular shoulder of a container in a clamping fit, comprises a circular bottom section which closes the container opening, and two head sections which project beyond the container. In between the head sections a slide is arranged which in a retracted initial state assumes, together with the head sections, a substantially frusta conical shape tapering to the upper side. The frusta conical edge has a maximum diameter which is at the most, as large as the outer diameter of the container at the container opening. The slide is adapted to be pushed forwards under the action of a force of a predetermined magnitude in one direction beyond the rim of the container. The projected slide can be gripped by a user to lever off the stopper from the container.