edge margin occurs at a distance spaced inwardly sufficiently from the edge by associating the hold down rib 49 with the cover 11 the diaphragm seal 15 and container 3 can have a completely symmetrical construction no matter where the spout 43 is angularly located with respect to the central axis a of the container 3 the diaphragm seal 15 will always be able to open most easily immediately under the spout and be held from opening at locations radially opposite the spout as liquid is drawn out of the container 3 by the child the air pressure in the container is gradually reduced drawing a vacuum in the container 3 makes it increasingly difficult to keep the diaphragm seal 15 open to break the vacuum the bottom wall 69 of the diaphragm seal 15 deflects down as vacuum is detected in the container 3 more specifically air pressure acting through the air hole 59 in the retainer 13 pushes the bottom wall 69 of the diaphragm seal 15 down so that the seal with the annular seat 61 on the underside of the retainer is eventually broken air can then pass from the air hole 59 in the retainer 13 to the air hole 71 in the center of the bottom wall 69 of the diaphragm seal 15 and thence to the interior of the container 3 this configuration is illustrated in fig3 the air holes 59 71 retainer base 53 and bottom wall 69 of the diaphragm seal 15 define an air passage for delivering air from outside the container 3 to the interior of the container the operation of the bottom wall 69 of the diaphragm seal 15 to release its seal with the annular seat 61 is entirely independent of the.