A overcap is provided which covers the opening of a container. The overcap, in one form, engages an exterior of the container around the rim and along an inwardly directed flange which defines an opening of the container. The overcap snappingly engages with the container to produce an audible indication that the overcap is firmly attached to the container and that air is being expelled from inside the container. In another form, the overcap includes an invertible dome which is transformable between an upward convex position and an upward concave position.