A closure for a container , said container having an opening to the interior of the container wherein contents may be stored and an outwardly projecting flange , said closure comprising: (A) a base for being installed on the container at the container opening , said base having an access passage through said base for permitting access to the container interior; and (B) a closing element operable on said base between a closed position for occluding said access passage and an open position for permitting communication with said access passage , and wherein said base has a skirt for attaching said closure to the container , said skirt having (i) an exterior surface , (ii) an interior surface , (iii) a lower end portion for being located laterally outwardly of the container flange when said base is installed at the container opening , said lower end portion defining an annular, lower end surface , and (iv) an annular, flexible retention member extending laterally inwardly from said lower end portion so as to define a connection therebetween, said flexible retention member having (a) an initial, undeflected configuration, (b) a deflected configuration rotated away from said initial, undeflected configuration about said connection , characterized in that said flexible retention member further comprises (c) a proximal end surface located at said connection , said proximal end surface and said lower end surface together defining a shoulder when said flexible retention member is in said undeflected configuration, wherein said proximal end surface of said flexible retention member in said undeflected configuration is normal to said lower end surface when viewed in a vertical cross-sectional plane taken through a central axis of said closure . (A) a base for being installed on the container at the container opening , said base having an access passage through said base for permitting access to the container interior; and (B) a closing element operable on said base between a closed position for occluding said access passage and an open position for permitting communication with said access passage , and wherein said base has a skirt for attaching said closure to the container , said skirt having (i) an exterior surface , (ii) an interior surface , (iii) a lower end portion for being located laterally outwardly of the container flange when said base is installed at the container opening , said lower end portion defining an annular, lower end surface , and (iv) an annular, flexible retention member extending laterally inwardly from said lower end portion so as to define a connection therebetween, said flexible retention member having (a) an initial, undeflected configuration, (b) a deflected configuration rotated away from said initial, undeflected configuration about said connection , (a) an initial, undeflected configuration, (b) a deflected configuration rotated away from said initial, undeflected configuration about said connection , characterized in that said flexible retention member further comprises (c) a proximal end surface located at said connection , said proximal end surface and said lower end surface together defining a shoulder when said flexible retention member is in said undeflected configuration, wherein said proximal end surface of said flexible retention member in said undeflected configuration is normal to said lower end surface when viewed in a vertical cross-sectional plane taken through a central axis of said closure . <EOS>