Abstract: A pressure-responsive container assembly and elastically-deformable penetrable cap is provided. Embodiments of the penetrable cap of the present invention include, but are not limited to: an annular sealing portion for engaging an inner surface of a container, a substantially rigid portion extending radially inward from the annular sealing portion; a flexible transition portion extending radially inward from the substantially rigid portion; and a penetrable portion extending radially inward from the transition portion for closing the opening defined by the container. Thus, embodiments of the present invention may thus allow the penetrable portion to elastically deform about the transition portion to a generally convex shape so as to exert a radially outward force that may be transmitted by the substantially rigid portion to the annular sealing portion so as to reinforce a fluid-tight seal between the annular sealing portion and the inner surface of the container.